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/web/sse-events.ts

@@ -84,6 +84,8 @@ export const SessionLimitResumeCancelled = 'session:limitResumeCancelled' as con
 export const SessionRespawnBreakerTripped = 'session:respawnBreakerTripped' as const;
 /** CLI version/model info detected from session output. */
 export const SessionCliInfo = 'session:cliInfo' as const;
+/** Session pin state changed (COD-139): pinned/unpinned in the session manager list. */
+export const SessionPinned = 'session:pinned' as const;
 /** General session message (e.g. status text). */
 export const SessionMessage = 'session:message' as const;
 /** Session entered interactive mode (claude or shell). */
@@ -402,6 +404,7 @@ export const SseEvent = {
   SessionLimitResumeCancelled,
   SessionRespawnBreakerTripped,
   SessionCliInfo,
+  SessionPinned,
   SessionMessage,
   SessionInteractive,
   SessionRunning,