fix(tmux-subagent): revert session.error cleanup (recoverable-error regression)

Oracle flagged a regression introduced in PR #3507 commit 21554be8:
event.ts routed session.error through tmux pane cleanup BEFORE the
existing session-recovery / model-fallback logic ran.

Problem: when session.error was recoverable (context window limit,
quota rate limit, provider fallback), the recovery/fallback code would
successfully continue the SAME session - but by then its tmux pane had
already been destroyed. User-visible symptom is exactly the original
complaint - 'screen appears but streaming stops working' after an
auto-retry.

Fix is the minimal revert: remove the onSessionError funnel from
event.ts and drop onSessionError from the manager. Fatal errors that
actually end a session still fire session.deleted, which continues to
trigger cleanup correctly. Non-fatal error streams stay attached to
the surviving pane.

src/features/tmux-subagent/manager.ts

@@ -838,18 +838,6 @@ export class TmuxSessionManager {
     await this.spawnQueue
   }
 
-  async onSessionError(event: { sessionID: string }): Promise<void> {
-    if (!this.isEnabled()) return
-    if (!this.getEffectiveSourcePaneId()) return
-    if (!this.sessions.has(event.sessionID)) return
-
-    log("[tmux-session-manager] onSessionError - routing to cleanup", {
-      sessionId: event.sessionID,
-    })
-
-    await this.onSessionDeleted(event)
-  }
-
   async onSessionDeleted(event: { sessionID: string }): Promise<void> {
     if (!this.isEnabled()) return
     if (!this.getEffectiveSourcePaneId()) return