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/history.go

@@ -1,9 +1,12 @@
 package gateway
 
 import (
+	"encoding/json"
 	"errors"
 	"net/http"
 	"strconv"
+	"strings"
+	"unicode/utf8"
 
 	"git.yeij.top/AskaEth/Cyrene_v2/internal/auth"
 	apperr "git.yeij.top/AskaEth/Cyrene_v2/internal/errors"
@@ -257,6 +260,57 @@ func (s *Server) handleDeleteSession(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, map[string]any{"deleted": id})
 }
 
+// renameSessionRequest is the PATCH /api/v1/sessions/{id} body.
+type renameSessionRequest struct {
+	Title string `json:"title"`
+}
+
+// maxSessionTitleRunes bounds a client-supplied session title so a display row never
+// overflows. It is a display-only bound: identity and permissions never derive from
+// the title (project red line).
+const maxSessionTitleRunes = 80
+
+// handleRenameSession renames a persistent session (main or sub). The title is
+// display-only metadata, so unlike DELETE it is allowed on the fixed main session. An
+// empty/whitespace title or one longer than maxSessionTitleRunes answers 400; an
+// unknown session id answers 404. It is owner/admin-gated like the other history
+// writes.
+func (s *Server) handleRenameSession(w http.ResponseWriter, r *http.Request) {
+	if s.history == nil {
+		writeError(w, &apperr.AppError{Code: "not_implemented", Message: "会话历史服务未配置（未注入 HistoryStore）", HTTPCode: http.StatusNotImplemented})
+		return
+	}
+	id := r.PathValue("id")
+	if id == "" {
+		writeError(w, apperr.Wrap(apperr.ErrBadRequest, "bad_request", "session id is required"))
+		return
+	}
+	var req renameSessionRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, apperr.Wrap(apperr.ErrBadRequest, "bad_request", "invalid request body"))
+		return
+	}
+	title := strings.TrimSpace(req.Title)
+	if title == "" {
+		writeError(w, apperr.Wrap(apperr.ErrBadRequest, "bad_request", "title is required"))
+		return
+	}
+	if utf8.RuneCountInString(title) > maxSessionTitleRunes {
+		writeError(w, apperr.Wrap(apperr.ErrBadRequest, "bad_request", "title is too long (max 80 characters)"))
+		return
+	}
+	if err := s.history.RenameSession(r.Context(), id, title); err != nil {
+		if errors.Is(err, history.ErrSessionNotFound) {
+			writeError(w, apperr.Wrap(apperr.ErrNotFound, "not_found", "session %q not found", id))
+			return
+		}
+		s.log.Error("history rename session", "err", err, "session", id)
+		writeError(w, apperr.Wrap(apperr.ErrInternal, "session_failed", "failed to rename session"))
+		return
+	}
+	writeJSON(w, http.StatusOK, map[string]any{"id": id, "title": title})
+}
+
 // toHistoryMessage maps a stored history.Message to its wire shape.
 func toHistoryMessage(m history.Message) historyMessage {
 	return historyMessage{