feat: add deployment-wide template allowlist for chats (#23262)

- Stores a deployment-wide agents template allowlist in `site_configs`
(`agents_template_allowlist`)
- Adds `GET/PUT /api/experimental/chats/config/template-allowlist`
endpoints
- Filters `list_templates`, `read_template`, and `create_workspace` chat
tools by allowlist, if defined (empty=all allowed)
- Add "Templates" admin settings tab in Agents UI ([what it looks
like](https://624de63c6aacee003aa84340-sitjilsyrr.chromatic.com/?path=/story/pages-agentspage-agentsettingspageview--template-allowlist))

> 🤖 This PR was created with the help of Coder Agents, and has been
reviewed by my human. 🧑‍💻

coderd/database/dbauthz/dbauthz.go

@@ -2674,6 +2674,17 @@ func (q *querier) GetChatSystemPrompt(ctx context.Context) (string, error) {
 	return q.db.GetChatSystemPrompt(ctx)
 }
 
+// GetChatTemplateAllowlist requires deployment-config read permission,
+// unlike the peer getters (GetChatDesktopEnabled, etc.) which only
+// check actor presence. The allowlist is admin-configuration that
+// should not be readable by non-admin users via the HTTP API.
+func (q *querier) GetChatTemplateAllowlist(ctx context.Context) (string, error) {
+	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceDeploymentConfig); err != nil {
+		return "", err
+	}
+	return q.db.GetChatTemplateAllowlist(ctx)
+}
+
 func (q *querier) GetChatUsageLimitConfig(ctx context.Context) (database.ChatUsageLimitConfig, error) {
 	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceDeploymentConfig); err != nil {
 		return database.ChatUsageLimitConfig{}, err
@@ -6812,6 +6823,13 @@ func (q *querier) UpsertChatSystemPrompt(ctx context.Context, value string) erro
 	return q.db.UpsertChatSystemPrompt(ctx, value)
 }
 
+func (q *querier) UpsertChatTemplateAllowlist(ctx context.Context, templateAllowlist string) error {
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
+		return err
+	}
+	return q.db.UpsertChatTemplateAllowlist(ctx, templateAllowlist)
+}
+
 func (q *querier) UpsertChatUsageLimitConfig(ctx context.Context, arg database.UpsertChatUsageLimitConfigParams) (database.ChatUsageLimitConfig, error) {
 	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
 		return database.ChatUsageLimitConfig{}, err

coderd/database/dbauthz/dbauthz_test.go

@@ -656,6 +656,10 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().GetChatDesktopEnabled(gomock.Any()).Return(false, nil).AnyTimes()
 		check.Args().Asserts()
 	}))
+	s.Run("GetChatTemplateAllowlist", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().GetChatTemplateAllowlist(gomock.Any()).Return("", nil).AnyTimes()
+		check.Args().Asserts(rbac.ResourceDeploymentConfig, policy.ActionRead)
+	}))
 	s.Run("GetChatWorkspaceTTL", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
 		dbm.EXPECT().GetChatWorkspaceTTL(gomock.Any()).Return("1h", nil).AnyTimes()
 		check.Args().Asserts()
@@ -873,6 +877,10 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().UpsertChatDesktopEnabled(gomock.Any(), false).Return(nil).AnyTimes()
 		check.Args(false).Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
 	}))
+	s.Run("UpsertChatTemplateAllowlist", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().UpsertChatTemplateAllowlist(gomock.Any(), "").Return(nil).AnyTimes()
+		check.Args("").Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
+	}))
 	s.Run("UpsertChatWorkspaceTTL", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
 		dbm.EXPECT().UpsertChatWorkspaceTTL(gomock.Any(), "1h").Return(nil).AnyTimes()
 		check.Args("1h").Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)