feat: separate default and additional Coder Agents system prompts (#23616)

Admins can now control whether the built-in Coder Agents default system
prompt is prepended to their custom instructions, rather than having the
custom prompt silently replace the default.

**Changes:**
- New `include_default_system_prompt` boolean toggle (defaults to `true`
for existing deployments) stored as a site config key — no migration
needed.
- GET `/api/experimental/chats/config/system-prompt` returns the toggle
state, the custom prompt, and a preview of the built-in default.
- PUT persists both the toggle and custom prompt atomically in a single
transaction.
- `resolvedChatSystemPrompt()` composes `[default?, custom?]` joined by
`\n\n`, falling back to the built-in default on DB errors.
- Settings UI adds a Switch toggle with conditional helper text and a
"Preview" button that shows the built-in default prompt via the existing
`TextPreviewDialog`.
- Comprehensive test coverage: 15 subtests covering toggle behavior,
prompt composition matrix, auth boundaries, and integration with chat
creation.

coderd/database/dbauthz/dbauthz.go

@@ -2578,6 +2578,18 @@ func (q *querier) GetChatFilesByIDs(ctx context.Context, ids []uuid.UUID) ([]dat
 	return files, nil
 }
 
+func (q *querier) GetChatIncludeDefaultSystemPrompt(ctx context.Context) (bool, error) {
+	// The include-default-system-prompt flag is a deployment-wide setting read
+	// during chat creation by every authenticated user, so no RBAC policy
+	// check is needed. We still verify that a valid actor exists in the
+	// context to ensure this is never callable by an unauthenticated or
+	// system-internal path without an explicit actor.
+	if _, ok := ActorFromContext(ctx); !ok {
+		return false, ErrNoActor
+	}
+	return q.db.GetChatIncludeDefaultSystemPrompt(ctx)
+}
+
 func (q *querier) GetChatMessageByID(ctx context.Context, id int64) (database.ChatMessage, error) {
 	// ChatMessages are authorized through their parent Chat.
 	// We need to fetch the message first to get its chat_id.
@@ -2674,6 +2686,18 @@ func (q *querier) GetChatSystemPrompt(ctx context.Context) (string, error) {
 	return q.db.GetChatSystemPrompt(ctx)
 }
 
+func (q *querier) GetChatSystemPromptConfig(ctx context.Context) (database.GetChatSystemPromptConfigRow, error) {
+	// The system prompt configuration is a deployment-wide setting read during
+	// chat creation by every authenticated user, so no RBAC policy check is
+	// needed. We still verify that a valid actor exists in the context to
+	// ensure this is never callable by an unauthenticated or system-internal
+	// path without an explicit actor.
+	if _, ok := ActorFromContext(ctx); !ok {
+		return database.GetChatSystemPromptConfigRow{}, ErrNoActor
+	}
+	return q.db.GetChatSystemPromptConfig(ctx)
+}
+
 // GetChatTemplateAllowlist requires deployment-config read permission,
 // unlike the peer getters (GetChatDesktopEnabled, etc.) which only
 // check actor presence. The allowlist is admin-configuration that
@@ -6842,6 +6866,13 @@ func (q *querier) UpsertChatDiffStatusReference(ctx context.Context, arg databas
 	return q.db.UpsertChatDiffStatusReference(ctx, arg)
 }
 
+func (q *querier) UpsertChatIncludeDefaultSystemPrompt(ctx context.Context, includeDefaultSystemPrompt bool) error {
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
+		return err
+	}
+	return q.db.UpsertChatIncludeDefaultSystemPrompt(ctx, includeDefaultSystemPrompt)
+}
+
 func (q *querier) UpsertChatSystemPrompt(ctx context.Context, value string) error {
 	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
 		return err

coderd/database/dbauthz/dbauthz_test.go

@@ -655,6 +655,17 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().GetChatQueuedMessages(gomock.Any(), chat.ID).Return(qms, nil).AnyTimes()
 		check.Args(chat.ID).Asserts(chat, policy.ActionRead).Returns(qms)
 	}))
+	s.Run("GetChatIncludeDefaultSystemPrompt", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().GetChatIncludeDefaultSystemPrompt(gomock.Any()).Return(true, nil).AnyTimes()
+		check.Args().Asserts()
+	}))
+	s.Run("GetChatSystemPromptConfig", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().GetChatSystemPromptConfig(gomock.Any()).Return(database.GetChatSystemPromptConfigRow{
+			ChatSystemPrompt:           "prompt",
+			IncludeDefaultSystemPrompt: true,
+		}, nil).AnyTimes()
+		check.Args().Asserts()
+	}))
 	s.Run("GetChatSystemPrompt", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
 		dbm.EXPECT().GetChatSystemPrompt(gomock.Any()).Return("prompt", nil).AnyTimes()
 		check.Args().Asserts()
@@ -900,6 +911,10 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().BackoffChatDiffStatus(gomock.Any(), arg).Return(nil).AnyTimes()
 		check.Args(arg).Asserts(rbac.ResourceChat, policy.ActionUpdate).Returns()
 	}))
+	s.Run("UpsertChatIncludeDefaultSystemPrompt", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().UpsertChatIncludeDefaultSystemPrompt(gomock.Any(), false).Return(nil).AnyTimes()
+		check.Args(false).Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
+	}))
 	s.Run("UpsertChatSystemPrompt", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
 		dbm.EXPECT().UpsertChatSystemPrompt(gomock.Any(), "").Return(nil).AnyTimes()
 		check.Args("").Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)