refactor: remove legacy live-read and injected-history chat context paths (#26585)

This PR makes the agent-pushed pinned snapshot
(`chat_context_resources`) the sole source of workspace context for
chats, completing the "Release 5" cleanup. It removes legacy mechanisms
now superseded by the snapshot that agents push over dRPC
(`PushContextState`) and refresh via `chat-context/refresh`.

Removed:

- **Live-read at turn time.** MCP tool discovery, skill live-body reads,
and the instruction/skill history fallback that dialed the workspace on
every turn.
- **Context injected as message history.** The
`persist_workspace_context` generation action and its decision-loop
guard.
- **The legacy write path.** `POST`/`DELETE
/api/v2/workspaceagents/me/experimental/chat-context`, the agentsdk
`AddChatContext`/`ClearChatContext` methods, and the CLI one-shot
writer.
- **The `chats.last_injected_context` column** and all of its plumbing
(migration `000529`, queries, `db2sdk`, `dbauthz`, audit table, and the
frontend `ContextUsageIndicator` fallback).

Subagent context inheritance no longer copies parent context messages;
children now hydrate the parent's pinned `chat_context_resources` on
create, which yields an identical pin for the same workspace and agent.

What stays (still served by the live agent connection, not the
snapshot): `read_skill_file` supporting-file reads, `read_skill`
supporting-file listing, and MCP tool execution.

> [!NOTE]
> Migration `000529` drops `chats.last_injected_context` and recreates
the `chats_expanded` view without it. The down migration restores both.

<details>
<summary>Decision log (D1-D5)</summary>

- **D1 (subagent inheritance):** Re-point inheritance from the legacy
message copy to a pinned hydrate. Children call
`hydrateChatContextOnCreate` instead of copying parent context messages.
- **D2 (`persist_workspace_context`):** Remove the generation action
entirely along with the decision-loop guard it existed to satisfy, since
context is never injected into history anymore.
- **D3 (legacy HTTP + CLI):** Remove the experimental `chat-context`
POST/DELETE endpoints, the agentsdk methods, and the CLI one-shot. The
dRPC push + `chat-context/refresh` replace them.
- **D4 (frontend fallback):** Remove the `last_injected_context`
fallback in `ContextUsageIndicator`; pinned `resources` are the sole
source.
- **D5 (sequencing):** Ship as a single PR rather than a stacked pair.

</details>

---
Coder Agents generated on behalf of @kylecarbs.

coderd/database/queries/chats.sql

@@ -29,7 +29,6 @@ chats_expanded AS (
         updated_chats.agent_id,
         updated_chats.pin_order,
         updated_chats.last_read_message_id,
-        updated_chats.last_injected_context,
         updated_chats.dynamic_tools,
         updated_chats.organization_id,
         updated_chats.plan_mode,
@@ -96,7 +95,6 @@ chats_expanded AS (
         updated_chats.agent_id,
         updated_chats.pin_order,
         updated_chats.last_read_message_id,
-        updated_chats.last_injected_context,
         updated_chats.dynamic_tools,
         updated_chats.organization_id,
         updated_chats.plan_mode,
@@ -762,7 +760,6 @@ chats_expanded AS (
         inserted_chat.agent_id,
         inserted_chat.pin_order,
         inserted_chat.last_read_message_id,
-        inserted_chat.last_injected_context,
         inserted_chat.dynamic_tools,
         inserted_chat.organization_id,
         inserted_chat.plan_mode,
@@ -911,7 +908,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -978,7 +974,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -1043,7 +1038,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -1108,7 +1102,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -1173,7 +1166,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -1237,7 +1229,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -1301,73 +1292,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
-        updated_chat.dynamic_tools,
-        updated_chat.organization_id,
-        updated_chat.plan_mode,
-        updated_chat.client_type,
-        updated_chat.last_turn_summary,
-        updated_chat.snapshot_version,
-        updated_chat.history_version,
-        updated_chat.queue_version,
-        updated_chat.generation_attempt,
-        updated_chat.retry_state,
-        updated_chat.retry_state_version,
-        updated_chat.runner_id,
-        updated_chat.requires_action_deadline_at,
-        COALESCE(root.user_acl, updated_chat.user_acl) AS user_acl,
-        COALESCE(root.group_acl, updated_chat.group_acl) AS group_acl,
-        owner.username AS owner_username,
-        owner.name AS owner_name,
-        updated_chat.context_aggregate_hash,
-        updated_chat.context_dirty_since,
-        updated_chat.context_dirty_resources,
-        updated_chat.context_error
-    FROM
-        updated_chat
-    LEFT JOIN chats root ON root.id = COALESCE(updated_chat.root_chat_id, updated_chat.parent_chat_id)
-    JOIN visible_users owner ON owner.id = updated_chat.owner_id
-)
-SELECT *
-FROM chats_expanded;
-
--- name: UpdateChatLastInjectedContext :one
-WITH updated_chat AS (
--- Updates the cached injected context parts (AGENTS.md +
--- skills) on the chat row. Called only when context changes
--- (first workspace attach or agent change). updated_at is
--- intentionally not touched to avoid reordering the chat list.
-UPDATE chats SET
-    last_injected_context = sqlc.narg('last_injected_context')::jsonb
-WHERE
-    id = @id::uuid
-RETURNING *
-),
-chats_expanded AS (
-    SELECT
-        updated_chat.id,
-        updated_chat.owner_id,
-        updated_chat.workspace_id,
-        updated_chat.title,
-        updated_chat.status,
-        updated_chat.worker_id,
-        updated_chat.started_at,
-        updated_chat.heartbeat_at,
-        updated_chat.created_at,
-        updated_chat.updated_at,
-        updated_chat.parent_chat_id,
-        updated_chat.root_chat_id,
-        updated_chat.last_model_config_id,
-        updated_chat.archived,
-        updated_chat.last_error,
-        updated_chat.mode,
-        updated_chat.mcp_server_ids,
-        updated_chat.labels,
-        updated_chat.build_id,
-        updated_chat.agent_id,
-        updated_chat.pin_order,
-        updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -1449,7 +1373,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -1666,7 +1589,6 @@ chats_expanded AS (
         acquired_chats.agent_id,
         acquired_chats.pin_order,
         acquired_chats.last_read_message_id,
-        acquired_chats.last_injected_context,
         acquired_chats.dynamic_tools,
         acquired_chats.organization_id,
         acquired_chats.plan_mode,
@@ -1735,7 +1657,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -1804,7 +1725,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -2080,7 +2000,6 @@ chats_expanded AS (
         locked_chat.agent_id,
         locked_chat.pin_order,
         locked_chat.last_read_message_id,
-        locked_chat.last_injected_context,
         locked_chat.dynamic_tools,
         locked_chat.organization_id,
         locked_chat.plan_mode,
@@ -2141,7 +2060,6 @@ chats_expanded AS (
         shared_chat.agent_id,
         shared_chat.pin_order,
         shared_chat.last_read_message_id,
-        shared_chat.last_injected_context,
         shared_chat.dynamic_tools,
         shared_chat.organization_id,
         shared_chat.plan_mode,
@@ -2821,7 +2739,6 @@ chats_expanded AS (
         bumped_chat.agent_id,
         bumped_chat.pin_order,
         bumped_chat.last_read_message_id,
-        bumped_chat.last_injected_context,
         bumped_chat.dynamic_tools,
         bumped_chat.organization_id,
         bumped_chat.plan_mode,
@@ -2893,7 +2810,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,
@@ -2957,7 +2873,6 @@ chats_expanded AS (
         updated_chat.agent_id,
         updated_chat.pin_order,
         updated_chat.last_read_message_id,
-        updated_chat.last_injected_context,
         updated_chat.dynamic_tools,
         updated_chat.organization_id,
         updated_chat.plan_mode,