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now superseded by the snapshot that agents push over dRPC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.

codersdk/agentsdk/agentsdk.go

@@ -990,37 +990,6 @@ func (s *SSEAgentReinitReceiver) Receive(ctx context.Context) (*Reinitialization
 	}
 }
 
-// AddChatContextRequest is the request body for adding chat context.
-type AddChatContextRequest struct {
-	// ChatID optionally identifies the chat to add context to.
-	// If empty, auto-detection is used (CODER_CHAT_ID env, the
-	// only active chat, or the only top-level active chat for this
-	// agent).
-	ChatID uuid.UUID `json:"chat_id,omitempty"`
-	// Parts are the context-file and skill parts to add.
-	Parts []codersdk.ChatMessagePart `json:"parts"`
-}
-
-// AddChatContextResponse is the response for adding chat context.
-type AddChatContextResponse struct {
-	ChatID uuid.UUID `json:"chat_id"`
-	Count  int       `json:"count"`
-}
-
-// ClearChatContextRequest is the request body for clearing chat context.
-type ClearChatContextRequest struct {
-	// ChatID optionally identifies the chat to clear context from.
-	// If empty, auto-detection is used (CODER_CHAT_ID env, the
-	// only active chat, or the only top-level active chat for this
-	// agent).
-	ChatID uuid.UUID `json:"chat_id,omitempty"`
-}
-
-// ClearChatContextResponse is the response for clearing chat context.
-type ClearChatContextResponse struct {
-	ChatID uuid.UUID `json:"chat_id"`
-}
-
 // RefreshChatContextResponse is the response for refreshing chat context.
 type RefreshChatContextResponse struct {
 	// Refreshed is the number of drifted chats that were re-pinned to the
@@ -1028,38 +997,6 @@ type RefreshChatContextResponse struct {
 	Refreshed int `json:"refreshed"`
 }
 
-// AddChatContext adds context-file and skill parts to an active chat.
-func (c *Client) AddChatContext(ctx context.Context, req AddChatContextRequest) (AddChatContextResponse, error) {
-	res, err := c.SDK.Request(ctx, http.MethodPost, "/api/v2/workspaceagents/me/experimental/chat-context", req)
-	if err != nil {
-		return AddChatContextResponse{}, xerrors.Errorf("execute request: %w", err)
-	}
-	defer res.Body.Close()
-
-	if res.StatusCode != http.StatusOK {
-		return AddChatContextResponse{}, codersdk.ReadBodyAsError(res)
-	}
-
-	var resp AddChatContextResponse
-	return resp, json.NewDecoder(res.Body).Decode(&resp)
-}
-
-// ClearChatContext soft-deletes context-file and skill messages from an active chat.
-func (c *Client) ClearChatContext(ctx context.Context, req ClearChatContextRequest) (ClearChatContextResponse, error) {
-	res, err := c.SDK.Request(ctx, http.MethodDelete, "/api/v2/workspaceagents/me/experimental/chat-context", req)
-	if err != nil {
-		return ClearChatContextResponse{}, xerrors.Errorf("execute request: %w", err)
-	}
-	defer res.Body.Close()
-
-	if res.StatusCode != http.StatusOK {
-		return ClearChatContextResponse{}, codersdk.ReadBodyAsError(res)
-	}
-
-	var resp ClearChatContextResponse
-	return resp, json.NewDecoder(res.Body).Decode(&resp)
-}
-
 // RefreshChatContext re-pins every drifted chat bound to this agent to the
 // agent's latest context snapshot, clearing their drift markers. It backs
 // the in-workspace `coder exp chat context refresh` (no chat argument),

codersdk/chats.go

@@ -137,11 +137,6 @@ type Chat struct {
 	// the owner's read cursor, which updates on stream
 	// connect and disconnect.
 	HasUnread bool `json:"has_unread"`
-	// LastInjectedContext holds the most recently persisted
-	// injected context parts (AGENTS.md files and skills). It
-	// is updated only when context changes, on first workspace
-	// attach or agent change.
-	LastInjectedContext []ChatMessagePart `json:"last_injected_context,omitempty"`
 	// Context reports the chat's pinned workspace-context state and
 	// whether it has drifted from the agent's latest pushed snapshot.
 	// Nil when the chat has no pinned context yet.