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/title.go

@@ -0,0 +1,86 @@
+package history
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// This file owns the two session-title paths: auto-naming a session from its first
+// user message and an explicit rename. A title is display-only metadata (design/16
+// §2.1) and is never used to identify a session, a sender or a permission — identity
+// always comes from the channel actor metadata / JWT (project red line).
+
+const (
+	// autoTitleMaxRunes bounds an auto-generated title so a session list row never
+	// overflows; a longer first message is truncated with an ellipsis.
+	autoTitleMaxRunes = 20
+	// titleMainFallback and titleSubFallback name a session whose first user
+	// message carries no text (e.g. an image-only turn), so the session list is
+	// never nameless.
+	titleMainFallback = "主会话"
+	titleSubFallback  = "次级会话"
+)
+
+// ErrSessionNotFound is returned by RenameSession when no chat_sessions row carries
+// the requested id.
+var ErrSessionNotFound = errors.New("history: session not found")
+
+// RenameSession sets the display title of sessionID. Both the fixed main session
+// (id "web") and sub sessions may be renamed: renaming is not a destructive identity
+// change, unlike deletion. An unknown id returns ErrSessionNotFound. The caller
+// validates the title (non-empty, length-bounded); the store persists it verbatim.
+func (s *Store) RenameSession(ctx context.Context, sessionID, title string) error {
+	res, err := s.db.ExecContext(ctx, `UPDATE chat_sessions SET title = ? WHERE id = ?`, title, sessionID)
+	if err != nil {
+		return fmt.Errorf("history: rename session %s: %w", sessionID, err)
+	}
+	n, err := res.RowsAffected()
+	if err != nil {
+		return fmt.Errorf("history: rename session %s rows: %w", sessionID, err)
+	}
+	if n == 0 {
+		return fmt.Errorf("history: rename session %s: %w", sessionID, ErrSessionNotFound)
+	}
+	return nil
+}
+
+// ensureTitle names a session from its first user message. It is a no-op unless the
+// session row exists in chat_sessions, its title is still empty, and no user message
+// has been persisted for it yet (so the caller must invoke it before inserting the
+// message). A message that is all whitespace falls back to the session-type label.
+// The title is display-only metadata, so a failure is logged and swallowed rather
+// than failing the message write it accompanies.
+func (s *Store) ensureTitle(ctx context.Context, sessionID, content string) {
+	if sessionID == "" {
+		return
+	}
+	title := deriveTitle(content)
+	_, err := s.db.ExecContext(ctx, `UPDATE chat_sessions
+		SET title = CASE WHEN ? = '' THEN CASE WHEN type = ? THEN ? ELSE ? END ELSE ? END
+		WHERE id = ? AND title = ''
+		  AND NOT EXISTS (SELECT 1 FROM chat_messages WHERE session_id = ? AND role = ?)`,
+		title, SessionMain, titleMainFallback, titleSubFallback, title,
+		sessionID, sessionID, RoleUser)
+	if err != nil && s.log != nil {
+		s.log.Warn("auto title failed", "err", err, "session_id", sessionID)
+	}
+}
+
+// deriveTitle turns a user message into a session title: whitespace runs (newlines
+// included) collapse to a single space, then the result is truncated to
+// autoTitleMaxRunes runes with an ellipsis. A message that is all whitespace yields
+// "" so ensureTitle falls back to the session-type label.
+func deriveTitle(content string) string {
+	fields := strings.Fields(content)
+	if len(fields) == 0 {
+		return ""
+	}
+	joined := strings.Join(fields, " ")
+	runes := []rune(joined)
+	if len(runes) <= autoTitleMaxRunes {
+		return joined
+	}
+	return string(runes[:autoTitleMaxRunes]) + "…"
+}