feat(history): 会话列表lastMessage预览(B3) — 单查询无N+1

- history/session_preview.go: SessionMeta.LastMessage + 单条SQL(LEFT JOIN取每会话ORDER BY created_at DESC,rowid DESC LIMIT 1,走覆盖索引)
- ListSessions 迁出重写(签名不变), content折叠空白截前60rune
- gateway: historySessionSummary 增 lastMessage{role,content,createdAt}(omitempty), /sessions与/admin/sessions共用
- 新增测试(session_preview_test, history_lastmessage_test)
- Docs/api-client.md v0.22 同步

internal/runtime/history/store.go

@@ -109,6 +109,11 @@ type SessionMeta struct {
 	Type      string // "main" | "sub"
 	Title     string
 	CreatedAt int64
+	// LastMessage is a display-only preview of the session's newest message, filled
+	// by ListSessions in the same query. It is nil for a session with no messages.
+	// A client uses it to render the conversation row without a per-session request;
+	// identity and permissions never derive from it.
+	LastMessage *MessageSummary
 }
 
 // HistoryStore abstracts the chat-history store so callers (the gateway) and tests can
@@ -481,36 +486,6 @@ func (s *Store) NewSubSession(ctx context.Context, ownerID string) (string, erro
 	return sessionID, nil
 }
 
-// ListSessions returns the main session first, then every sub session, oldest first. The
-// returned slice is non-nil.
-func (s *Store) ListSessions(ctx context.Context) ([]SessionMeta, error) {
-	rows, err := s.db.QueryContext(ctx, `SELECT id, type, title, created_at FROM chat_sessions
-		ORDER BY (type = ?) DESC, created_at ASC`, SessionMain)
-	if err != nil {
-		return nil, fmt.Errorf("history: list sessions: %w", err)
-	}
-	defer func() { _ = rows.Close() }()
-
-	sessions := make([]SessionMeta, 0)
-	for rows.Next() {
-		var (
-			meta      SessionMeta
-			typeStr   string
-			createdAt int64
-		)
-		if err := rows.Scan(&meta.ID, &typeStr, &meta.Title, &createdAt); err != nil {
-			return nil, fmt.Errorf("history: scan session: %w", err)
-		}
-		meta.Type = typeStr
-		meta.CreatedAt = createdAt
-		sessions = append(sessions, meta)
-	}
-	if err := rows.Err(); err != nil {
-		return nil, fmt.Errorf("history: iterate sessions: %w", err)
-	}
-	return sessions, nil
-}
-
 // ensureMainSession inserts the fixed main session row when it is absent, so ListSessions
 // always reports the main session even before any message is written.
 func (s *Store) ensureMainSession(ctx context.Context) error {