feat: add last_injected_context column to chats table (#23798)

Adds a nullable JSONB column `last_injected_context` to the `chats`
table that stores the most recently persisted injected context parts
(AGENTS.md context-file and skill message parts). The column is updated
only when `persistInstructionFiles()` runs — on first workspace attach
or when the agent changes — so there are no redundant writes on
subsequent turns.

Internal fields (`ContextFileContent`, `ContextFileOS`,
`ContextFileDirectory`, `SkillDir`) are stripped at write time so the
column only holds small metadata. No stripping needed on the read path.

<details>
<summary>Implementation notes</summary>

- New migration `000456` adds nullable `last_injected_context JSONB`
column.
- New SQL query `UpdateChatLastInjectedContext` writes the column
without touching `updated_at`.
- `persistInstructionFiles()` strips internal fields from parts via
`StripInternal()` before persisting.
- Sentinel path (no AGENTS.md) persists skill-only parts when skills
exist.
- `codersdk.Chat` exposes `LastInjectedContext []ChatMessagePart`
(omitempty).
- `db2sdk.Chat()` passes through the already-clean data.

</details>

coderd/database/db2sdk/db2sdk.go

@@ -1572,6 +1572,17 @@ func Chat(c database.Chat, diffStatus *database.ChatDiffStatus) codersdk.Chat {
 		convertedDiffStatus := ChatDiffStatus(c.ID, diffStatus)
 		chat.DiffStatus = &convertedDiffStatus
 	}
+	if c.LastInjectedContext.Valid {
+		var parts []codersdk.ChatMessagePart
+		// Internal fields are stripped at write time in
+		// chatd.updateLastInjectedContext, so no
+		// StripInternal call is needed here. Unmarshal
+		// errors are suppressed — the column is written by
+		// us with a known schema.
+		if err := json.Unmarshal(c.LastInjectedContext.RawMessage, &parts); err == nil {
+			chat.LastInjectedContext = parts
+		}
+	}
 	return chat
 }
 

coderd/database/db2sdk/db2sdk_test.go

@@ -541,6 +541,13 @@ func TestChat_AllFieldsPopulated(t *testing.T) {
 		PinOrder:          1,
 		MCPServerIDs:      []uuid.UUID{uuid.New()},
 		Labels:            database.StringMap{"env": "prod"},
+		LastInjectedContext: pqtype.NullRawMessage{
+			// Use a context-file part to verify internal
+			// fields are not present (they are stripped at
+			// write time by chatd, not at read time).
+			RawMessage: json.RawMessage(`[{"type":"context-file","context_file_path":"/AGENTS.md"}]`),
+			Valid:      true,
+		},
 	}
 	// Only ChatID is needed here. This test checks that
 	// Chat.DiffStatus is non-nil, not that every DiffStatus