feat(coderd): copy agent context resources into the per-chat pin (#26438)

## What

Populates `chat_context_resources` (the per-chat pinned copy added in
#26430) by copying from `workspace_agent_context_resources` at the
points where a chat's `context_aggregate_hash` is set, in the same
transaction, so the pinned hash and pinned bodies always agree. No
prompt-building change yet; consuming the pinned copy in
`prepareGeneration` is a later, experiment-gated PR.

## How

- `HydrateAgentChatsContext` now hydrates NULL-hash chats **and** copies
the agent's resources onto them in one statement (a data-modifying CTE),
so the chat-create and agent-push paths need no Go change.
- New queries `InsertAgentContextResourcesIntoChat`,
`DeleteChatContextResources`, `ListChatContextResources`, each with a
hand-written dbauthz wrapper (per-chat update/read) and a
`MethodTestSuite` entry.
- `RefreshChatContext` re-pins resources via a shared `repinChatContext`
helper (clear-then-copy in a transaction). A dirty chat keeps its old
bodies until refresh.
- On agent rebind (e.g. a workspace rebuild produces a new agent), the
chat's context is re-pinned to the new agent so it stops injecting the
previous agent's resources. Best-effort: a context error never fails the
binding.

## Invariant

A chat's `chat_context_resources` always correspond to its
`context_aggregate_hash`. Bodies are (re)written only when the hash is
set (hydrate, refresh, rebind); a dirty chat keeps its old bodies until
refresh.

## Testing

Extends the context integration test to push real resources and assert
the copy across hydrate, dirty (no re-copy), and refresh. The dbauthz
`MethodTestSuite` covers the three new methods.

<details>
<summary>Why clear-then-copy (two statements)</summary>

The refresh/rebind re-pin clears the chat's rows then inserts the
agent's. It uses two sequential statements inside the transaction rather
than a single `WITH cleared AS (DELETE ...) INSERT ...`, because a
data-modifying CTE cannot see its own delete under snapshot isolation,
so overlapping sources (the common case: the same files re-pinned) would
collide on the `(chat_id, source)` primary key. The hydrate path inserts
into never-pinned (NULL-hash) chats and uses `ON CONFLICT DO UPDATE`
defensively.

</details>

<details>
<summary>Follow-ups</summary>

- `prepareGeneration` consuming the pinned instructions and skills
(experiment-gated).
- `codersdk.ChatContext` resources plus changed diff, and the frontend
indicator/refresh.
- Removing the per-turn pull and `last_injected_context`.

</details>

---

*This PR was created by Coder Agents on behalf of @kylecarbs.* Builds on
#26430.

coderd/database/dbauthz/dbauthz.go

@@ -2065,6 +2065,17 @@ func (q *querier) DeleteApplicationConnectAPIKeysByUserID(ctx context.Context, u
 	return q.db.DeleteApplicationConnectAPIKeysByUserID(ctx, userID)
 }
 
+func (q *querier) DeleteChatContextResourcesByChatID(ctx context.Context, chatID uuid.UUID) error {
+	chat, err := q.db.GetChatByID(ctx, chatID)
+	if err != nil {
+		return err
+	}
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, chat); err != nil {
+		return err
+	}
+	return q.db.DeleteChatContextResourcesByChatID(ctx, chatID)
+}
+
 func (q *querier) DeleteChatDebugDataAfterMessageID(ctx context.Context, arg database.DeleteChatDebugDataAfterMessageIDParams) (int64, error) {
 	chat, err := q.db.GetChatByID(ctx, arg.ChatID)
 	if err != nil {
@@ -5767,6 +5778,17 @@ func (q *querier) InsertAPIKey(ctx context.Context, arg database.InsertAPIKeyPar
 		q.db.InsertAPIKey)(ctx, arg)
 }
 
+func (q *querier) InsertAgentContextResourcesIntoChat(ctx context.Context, arg database.InsertAgentContextResourcesIntoChatParams) error {
+	chat, err := q.db.GetChatByID(ctx, arg.ChatID)
+	if err != nil {
+		return err
+	}
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, chat); err != nil {
+		return err
+	}
+	return q.db.InsertAgentContextResourcesIntoChat(ctx, arg)
+}
+
 func (q *querier) InsertAllUsersGroup(ctx context.Context, organizationID uuid.UUID) (database.Group, error) {
 	// This method creates a new group.
 	return insert(q.log, q.auth, rbac.ResourceGroup.InOrg(organizationID), q.db.InsertAllUsersGroup)(ctx, organizationID)
@@ -6549,6 +6571,17 @@ func (q *querier) ListBoundaryLogsBySessionID(ctx context.Context, arg database.
 	return q.db.ListBoundaryLogsBySessionID(ctx, arg)
 }
 
+func (q *querier) ListChatContextResourcesByChatID(ctx context.Context, chatID uuid.UUID) ([]database.ChatContextResource, error) {
+	chat, err := q.db.GetChatByID(ctx, chatID)
+	if err != nil {
+		return nil, err
+	}
+	if err := q.authorizeContext(ctx, policy.ActionRead, chat); err != nil {
+		return nil, err
+	}
+	return q.db.ListChatContextResourcesByChatID(ctx, chatID)
+}
+
 func (q *querier) ListChatUsageLimitGroupOverrides(ctx context.Context) ([]database.ListChatUsageLimitGroupOverridesRow, error) {
 	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceDeploymentConfig); err != nil {
 		return nil, err

coderd/database/dbauthz/dbauthz_test.go

@@ -551,6 +551,26 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().SetChatContextSnapshot(gomock.Any(), arg).Return(nil).AnyTimes()
 		check.Args(arg).Asserts(chat, policy.ActionUpdate)
 	}))
+	s.Run("InsertAgentContextResourcesIntoChat", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		chat := testutil.Fake(s.T(), faker, database.Chat{})
+		arg := database.InsertAgentContextResourcesIntoChatParams{ChatID: chat.ID, AgentID: uuid.New()}
+		dbm.EXPECT().GetChatByID(gomock.Any(), chat.ID).Return(chat, nil).AnyTimes()
+		dbm.EXPECT().InsertAgentContextResourcesIntoChat(gomock.Any(), arg).Return(nil).AnyTimes()
+		check.Args(arg).Asserts(chat, policy.ActionUpdate)
+	}))
+	s.Run("DeleteChatContextResourcesByChatID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		chat := testutil.Fake(s.T(), faker, database.Chat{})
+		dbm.EXPECT().GetChatByID(gomock.Any(), chat.ID).Return(chat, nil).AnyTimes()
+		dbm.EXPECT().DeleteChatContextResourcesByChatID(gomock.Any(), chat.ID).Return(nil).AnyTimes()
+		check.Args(chat.ID).Asserts(chat, policy.ActionUpdate).Returns()
+	}))
+	s.Run("ListChatContextResourcesByChatID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
+		chat := testutil.Fake(s.T(), faker, database.Chat{})
+		rows := []database.ChatContextResource{testutil.Fake(s.T(), faker, database.ChatContextResource{ChatID: chat.ID})}
+		dbm.EXPECT().GetChatByID(gomock.Any(), chat.ID).Return(chat, nil).AnyTimes()
+		dbm.EXPECT().ListChatContextResourcesByChatID(gomock.Any(), chat.ID).Return(rows, nil).AnyTimes()
+		check.Args(chat.ID).Asserts(chat, policy.ActionRead).Returns(rows)
+	}))
 	s.Run("GetChatWorkerAcquisitionCandidates", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		arg := database.GetChatWorkerAcquisitionCandidatesParams{
 			StaleSeconds: 30,