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/runtime/history/store.go

@@ -151,6 +151,10 @@ type HistoryStore interface {
 	// DeleteSession removes a sub session and all its messages. The main session is
 	// immutable and returns ErrImmutableMain.
 	DeleteSession(ctx context.Context, sessionID string) error
+	// RenameSession sets a session's display title. Both the main session and sub
+	// sessions may be renamed (a title is display-only metadata); an unknown id
+	// returns ErrSessionNotFound.
+	RenameSession(ctx context.Context, sessionID, title string) error
 	// NewSubSession creates a new sub session and returns its id (MainSessionID:uuid).
 	NewSubSession(ctx context.Context, ownerID string) (string, error)
 	// ListSessions returns the main session first then every sub session, oldest first.
@@ -229,6 +233,9 @@ func (s *Store) insert(ctx context.Context, msg Message) (Message, error) {
 	if msg.Platform == "" {
 		msg.Platform = PlatformWeb
 	}
+	if msg.Role == RoleUser {
+		s.ensureTitle(ctx, msg.SessionID, msg.Content)
+	}
 	_, err := s.db.ExecContext(ctx, `INSERT INTO chat_messages
 		(id, session_id, role, content, mode, mood, image, thinking, platform, device_id, sender_name, adapter_id, created_at)
 		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,