feat(gateway): 会话title生成+PATCH改名(A4) — 首条用户消息自动命名+可重命名

- history/title.go: RenameSession(改名字段)+ensureTitle(首条用户消息自动命名,折叠空白取前20rune,纯空白回落主会话/次级会话)
- store.go insert() 前调用 ensureTitle(覆盖Web+IM,最小侵入), HistoryStore 新增 RenameSession
- gateway: PATCH /api/v1/sessions/{id} (owner/admin, ≤80rune, 主会话可改名), handleRenameSession
- 新增/扩展测试(title_test, history_handler_test, fake同步)
- Docs/api-client.md v0.21 同步

internal/services/gateway/server.go

@@ -119,6 +119,9 @@ type HistoryStore interface {
 	ListAdapterLog(ctx context.Context, adapterID string, before int64, limit int) ([]history.Message, error)
 	ListAllLog(ctx context.Context, before int64, limit int) ([]history.Message, error)
 	DeleteSession(ctx context.Context, sessionID string) error
+	// RenameSession sets a session's display title (main or sub). An unknown id
+	// returns history.ErrSessionNotFound.
+	RenameSession(ctx context.Context, sessionID, title string) error
 	NewSubSession(ctx context.Context, ownerID string) (string, error)
 	ListSessions(ctx context.Context) ([]history.SessionMeta, error)
 }
@@ -400,6 +403,7 @@ func (s *Server) Handler() http.Handler {
 	mux.Handle("GET /api/v1/sessions/{id}/messages", historyAuth(http.HandlerFunc(s.handleListMessages)))
 	mux.Handle("POST /api/v1/sessions", historyAuth(http.HandlerFunc(s.handleCreateSubSession)))
 	mux.Handle("DELETE /api/v1/sessions/{id}", historyAuth(http.HandlerFunc(s.handleDeleteSession)))
+	mux.Handle("PATCH /api/v1/sessions/{id}", historyAuth(http.HandlerFunc(s.handleRenameSession)))
 
 	// Realtime.
 	mux.HandleFunc("/ws/chat", s.handleWS)