COD-139 add session pinning (float pinned sessions to top of session manager list)

Pin/unpin a session via POST /api/sessions/:id/pin {pinned}; pinned sessions
sort above unpinned in the unified session manager list (COD-121), ordered by
pinnedAt descending. Pin state lives on SessionState, persists to state.json,
and survives reload/reconnect/restart (persisted-input carries pinned; the
merge skips undefined so a recovered live session can't clobber it). New SSE
event session:pinned re-sorts the open list live across clients. Pin/Unpin
affordance in the session-row kebab menu with a 📌 glyph + amber highlight.

(cherry picked from commit 82749747039afcd4a3104f6a97ce7d3c2ddd048d)

src/types/session.ts

@@ -241,6 +241,10 @@ export interface SessionState {
   autoResumeEnabled?: boolean;
   /** Pending usage-limit auto-resume fire time (epoch ms), if armed */
   autoResumeAt?: number;
+  /** Pinned to the top of the session manager list (COD-139) */
+  pinned?: boolean;
+  /** When the session was pinned (epoch ms) — orders the pinned group, most-recent-first */
+  pinnedAt?: number;
   /** Image watcher enabled for this session */
   imageWatcherEnabled?: boolean;
   /** Total cost in USD */