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now superseded by the snapshot that agents push over dRPC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/workspaceagents_active_chat_internal_test.go

@@ -1,7 +1,7 @@
 package coderd
 
 import (
-	"fmt"
+	"database/sql"
 	"testing"
 
 	"github.com/google/uuid"
@@ -16,66 +16,6 @@ import (
 	"github.com/coder/coder/v2/testutil"
 )
 
-func TestActiveAgentChatDefinitionsAgree(t *testing.T) {
-	t.Parallel()
-
-	ctx := dbauthz.AsSystemRestricted(testutil.Context(t, testutil.WaitMedium))
-	db, _ := dbtestutil.NewDB(t)
-
-	org, err := db.GetDefaultOrganization(ctx)
-	require.NoError(t, err)
-
-	owner := dbgen.User(t, db, database.User{})
-	workspace := dbfake.WorkspaceBuild(t, db, database.WorkspaceTable{
-		OrganizationID: org.ID,
-		OwnerID:        owner.ID,
-	}).WithAgent().Do()
-	modelConfig := insertAgentChatTestModelConfig(t, db, owner.ID)
-
-	insertedChats := make([]database.Chat, 0, len(database.AllChatStatusValues())*2)
-	for _, archived := range []bool{false, true} {
-		for _, status := range database.AllChatStatusValues() {
-			chat := dbgen.Chat(t, db, database.Chat{
-				OrganizationID:    org.ID,
-				Status:            status,
-				OwnerID:           owner.ID,
-				LastModelConfigID: modelConfig.ID,
-				Title:             fmt.Sprintf("%s-archived-%t", status, archived),
-				AgentID:           uuid.NullUUID{UUID: workspace.Agents[0].ID, Valid: true},
-			})
-
-			if archived {
-				_, err = db.ArchiveChatByID(ctx, chat.ID)
-				require.NoError(t, err)
-
-				chat, err = db.GetChatByID(ctx, chat.ID)
-				require.NoError(t, err)
-			}
-
-			insertedChats = append(insertedChats, chat)
-		}
-	}
-
-	activeChats, err := db.GetActiveChatsByAgentID(ctx, workspace.Agents[0].ID)
-	require.NoError(t, err)
-
-	activeByID := make(map[uuid.UUID]bool, len(activeChats))
-	for _, chat := range activeChats {
-		activeByID[chat.ID] = true
-	}
-
-	for _, chat := range insertedChats {
-		require.Equalf(
-			t,
-			isActiveAgentChat(chat),
-			activeByID[chat.ID],
-			"status=%s archived=%t",
-			chat.Status,
-			chat.Archived,
-		)
-	}
-}
-
 func TestActiveAgentChatsIncludeInheritedACLs(t *testing.T) {
 	t.Parallel()
 
@@ -157,3 +97,31 @@ func TestActiveAgentChatsIncludeInheritedACLs(t *testing.T) {
 	require.Equal(t, rootUserACL, fetchedChild.UserACL)
 	require.Equal(t, rootGroupACL, fetchedChild.GroupACL)
 }
+
+func insertAgentChatTestModelConfig(
+	t testing.TB,
+	db database.Store,
+	userID uuid.UUID,
+) database.ChatModelConfig {
+	t.Helper()
+
+	createdBy := uuid.NullUUID{UUID: userID, Valid: true}
+
+	provider := dbgen.AIProvider(t, db, database.AIProvider{
+		Type:        database.AIProviderTypeOpenai,
+		Name:        "test-openai",
+		DisplayName: sql.NullString{String: "OpenAI", Valid: true},
+	})
+	dbgen.AIProviderKey(t, db, database.AIProviderKey{
+		ProviderID: provider.ID,
+		APIKey:     "test-api-key",
+	})
+
+	return dbgen.ChatModelConfig(t, db, database.ChatModelConfig{
+		Provider:     "openai",
+		AIProviderID: uuid.NullUUID{UUID: provider.ID, Valid: true},
+		CreatedBy:    createdBy,
+		UpdatedBy:    createdBy,
+		IsDefault:    true,
+	})
+}