feat: persist agent-pushed workspace context snapshots in coderd (#26145)

Replaces the v2.10 `PushContextState` stub with a real coderd write
path. Phase 1 of the chat-side persistence story; nothing reads these
rows yet.

Follows [#25983](https://github.com/coder/coder/pull/25983) and unblocks
[CODAGT-569](https://linear.app/codercom/issue/CODAGT-569/enable-agent-api-v210-pushcontextstate-bump-currentminor-wire-coderd).

## What ships

### Schema (`000517_workspace_agent_context.{up,down}.sql`)

Two new tables plus `api_key_scope` enum extensions:

- `workspace_agent_context_snapshots` (PK `workspace_agent_id` to
`workspace_agents(id) ON DELETE CASCADE`): one row per agent,
overwritten per push. Holds `version`, `schema_version`,
`aggregate_hash`, `snapshot_error`, `received_at`.
- `workspace_agent_context_resources` (PK `(workspace_agent_id,
source)`): per-resource state. `body_kind` and `status` are `TEXT` +
`CHECK` so adding new wire kinds (the RFC's reserved
PLUGIN/HOOK/SUBAGENT/COMMAND) is a one-line CHECK update plus a Go
switch case.

### SQLC queries (`coderd/database/queries/workspaceagentcontext.sql`)

- `UpsertWorkspaceAgentContextSnapshot`
- `UpsertWorkspaceAgentContextResource`
- `DeleteStaleWorkspaceAgentContextResources`
(delete-where-source-not-in)
- `GetLatestWorkspaceAgentContextSnapshot`
- `ListWorkspaceAgentContextResources`

### Handler (`coderd/agentapi/context.go`)

`ContextAPI` is a new sub-API. `PushContextState`:

1. Rejects `schema_version > 1` with a non-`Unimplemented` error so a
forward-incompatible agent fails loudly during rollout instead of
slipping into the permanent fallback path the `Unimplemented`
translation reserves for old coderd deployments.
2. Validates resources: no empty/duplicate sources, every variant maps
to a known body kind, every status maps to a known enum value, the
`Body` oneof is set (even when status is non-OK, mirroring the wire
guarantee so coderd can attribute failures to a known kind).
3. Inside `Database.InTx`, reads the existing snapshot. If the push is
not `initial` and `version` is not strictly greater, returns `accepted =
false` and leaves stored state untouched. Otherwise upserts the snapshot
row, upserts each resource, then runs the stale-source prune so the
snapshot and resource rows always agree.
4. Returns `accepted = true` on success.

Resource bodies are stored as `protojson(body oneof variant)` in `body
JSONB` with `body_kind` as the discriminator. Adding a new field to an
existing variant is zero work since `protojson` tolerates new fields;
adding a new variant is a CHECK + switch case.

### RBAC + dbauthz

- New `ResourceWorkspaceAgentContext` (Create/Read/Update/Delete).
- New `SubjectTypeAgentContext` plus `subjectAgentContext` system role
and `dbauthz.AsAgentContext` helper. The push handler elevates to this
subject; the agent's own role does not get direct write access to the
table.
- New `workspace_agent_context:*` API key scopes registered in the enum
migration; internal-only (not added to `externalLowLevel`).

### Audit

These rows are agent-pushed state, not user-authored. They are
intentionally not added to `AuditActionMap` and not enumerated in
`enterprise/audit/table.go`, matching `boundary_logs`,
`workspace_agent_memory_resource_monitor`, etc. `enterprise/audit` tests
pass unchanged.

## Tests

- `coderd/agentapi/context_test.go`: 12 subtests covering
accepts/rejects (schema version, empty/duplicate source, unknown status,
missing body), version semantics (stale dropped, same-version replay
dropped, `initial=true` overwrites lower version), variant coverage,
non-OK status persistence, and the empty-active-set prune case.
- `coderd/database/dbauthz/dbauthz_test.go`: 5 `MethodTestSuite` cases
covering the new queries.
- `coderd/rbac/roles_test.go`: `WorkspaceAgentContext` permission row
asserting no human role currently has access.
-
`coderd/database/migrations/testdata/fixtures/000517_workspace_agent_context.up.sql`:
one snapshot + one resource per known body kind plus a non-OK status, so
the migration test suite never lands with these tables empty.

## Out of scope (later phases)

- Chat hydration (`chats.context_aggregate_hash`,
`last_injected_context`).
- Dirty-bit fan-out and `PUT /chats/{id}/context`.
- Agent-side `POST /api/v0/context/resync` barrier and the `coder exp
chat context` CLI.
- `codersdk` chat-context wire types and the dashboard Sources drawer.
- Removal of the chatd per-turn pull fallback.

## Compat property

This is a pure write path. If anything here returns errors the agent's
`RunPush` loop backs off, no chat behavior changes, and the workspace
keeps behaving exactly like it did before v2.10.

<details>
<summary>Implementation plan and decision log</summary>

Key design calls:

1. **Concurrency**: Accept iff `req.Initial || req.Version >
existing.Version`. The strict RFC reading ("version comparison is
authoritative") locks restarted agents out because their per-process
counter resets to 1; honoring `initial=true` reflects the real reboot
reality while still rejecting steady-state replays/out-of-order pushes.
2. **Body encoding**: `protojson` over the oneof variant body proto,
stored in JSONB with `body_kind` discriminator. Structured at the API/Go
layer, schema-tolerant at the storage layer, and Phase 2 readers
round-trip back via `protojson.Unmarshal`.
3. **Schema version rejection**: returns a normal error, not
`Unimplemented`. The agent's `RunPush` loop only short-circuits on
`Unimplemented`; that escape hatch is reserved for old coderd
deployments. A forward-incompatible agent should retry-and-back-off, not
flip the connection into permanent fallback.
4. **Validation strictness**: empty sources, duplicate sources,
`STATUS_UNSPECIFIED`, and missing `Body` oneof variants are rejected
before any write so a misbehaving agent cannot poison the snapshot
table. Phase 2 readers can trust every row maps to a known proto
variant.

</details>

_This PR was authored by Coder Agents on Kyle Carberry's behalf._

coderd/database/queries/workspaceagents.sql

@@ -514,14 +514,28 @@ WHERE
 	AND deleted = FALSE;
 
 -- name: DeleteWorkspaceSubAgentByID :exec
-UPDATE
-	workspace_agents
-SET
-	deleted = TRUE
-WHERE
-	id = $1
-	AND parent_id IS NOT NULL
-	AND deleted = FALSE;
+-- Soft-deletes a single sub-agent (a child agent such as a devcontainer
+-- agent). Called from the DeleteSubAgent RPC when a sub-agent is torn
+-- down, which can happen mid-build without a full workspace rebuild.
+--
+-- Agent context rows are hard-deleted for the same reason as in
+-- SoftDeletePriorWorkspaceAgents: they only describe live agents, the
+-- rebuild-time soft-delete queries skip already-deleted agents, and
+-- agents are never hard-deleted, so the rows would otherwise orphan
+-- forever.
+WITH soft_deleted_agents AS (
+    UPDATE workspace_agents
+    SET deleted = TRUE
+    WHERE id = @id
+        AND parent_id IS NOT NULL
+        AND deleted = FALSE
+    RETURNING id
+), purged_context_resources AS (
+    DELETE FROM workspace_agent_context_resources
+    WHERE workspace_agent_id IN (SELECT id FROM soft_deleted_agents)
+)
+DELETE FROM workspace_agent_context_snapshots
+WHERE workspace_agent_id IN (SELECT id FROM soft_deleted_agents);
 
 -- name: GetWorkspaceAgentsForMetrics :many
 SELECT
@@ -577,17 +591,30 @@ LIMIT 1;
 -- provisionerdserver when a workspace build completes, after the new
 -- build's agents have been inserted, so running agents are not
 -- deleted while a build is still queued or provisioning.
-UPDATE workspace_agents
-SET deleted = TRUE
-WHERE id IN (
-    SELECT wa.id
-    FROM workspace_agents wa
-    JOIN workspace_resources wr ON wr.id = wa.resource_id
-    JOIN workspace_builds wb ON wb.job_id = wr.job_id
-    WHERE wb.workspace_id = @workspace_id
-      AND wb.id <> @current_build_id
-      AND wa.deleted = FALSE
-);
+--
+-- Agent context rows (workspace_agent_context_snapshots and
+-- workspace_agent_context_resources) only describe live agents, and
+-- agents are never un-deleted, so they are hard-deleted here instead
+-- of accumulating alongside the soft-deleted agent rows.
+WITH soft_deleted_agents AS (
+    UPDATE workspace_agents
+    SET deleted = TRUE
+    WHERE id IN (
+        SELECT wa.id
+        FROM workspace_agents wa
+        JOIN workspace_resources wr ON wr.id = wa.resource_id
+        JOIN workspace_builds wb ON wb.job_id = wr.job_id
+        WHERE wb.workspace_id = @workspace_id
+          AND wb.id <> @current_build_id
+          AND wa.deleted = FALSE
+    )
+    RETURNING id
+), purged_context_resources AS (
+    DELETE FROM workspace_agent_context_resources
+    WHERE workspace_agent_id IN (SELECT id FROM soft_deleted_agents)
+)
+DELETE FROM workspace_agent_context_snapshots
+WHERE workspace_agent_id IN (SELECT id FROM soft_deleted_agents);
 
 -- name: SoftDeleteWorkspaceAgentsByWorkspaceID :exec
 -- Marks every non-deleted agent belonging to the given workspace as
@@ -595,13 +622,24 @@ WHERE id IN (
 -- itself is soft-deleted, so the agent instance-identity auth path
 -- (which filters on workspace_agents.deleted) doesn't keep seeing
 -- orphaned rows.
-UPDATE workspace_agents
-SET deleted = TRUE
-WHERE id IN (
-    SELECT wa.id
-    FROM workspace_agents wa
-    JOIN workspace_resources wr ON wr.id = wa.resource_id
-    JOIN workspace_builds wb ON wb.job_id = wr.job_id
-    WHERE wb.workspace_id = @workspace_id
-      AND wa.deleted = FALSE
-);
+--
+-- Agent context rows are hard-deleted for the same reason as in
+-- SoftDeletePriorWorkspaceAgents.
+WITH soft_deleted_agents AS (
+    UPDATE workspace_agents
+    SET deleted = TRUE
+    WHERE id IN (
+        SELECT wa.id
+        FROM workspace_agents wa
+        JOIN workspace_resources wr ON wr.id = wa.resource_id
+        JOIN workspace_builds wb ON wb.job_id = wr.job_id
+        WHERE wb.workspace_id = @workspace_id
+          AND wa.deleted = FALSE
+    )
+    RETURNING id
+), purged_context_resources AS (
+    DELETE FROM workspace_agent_context_resources
+    WHERE workspace_agent_id IN (SELECT id FROM soft_deleted_agents)
+)
+DELETE FROM workspace_agent_context_snapshots
+WHERE workspace_agent_id IN (SELECT id FROM soft_deleted_agents);