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/plugin/event.ts

@@ -615,10 +615,6 @@ export function createEventHandler(args: {
         const sessionID = props?.sessionID as string | undefined;
         const error = props?.error;
 
-        if (tmuxIntegrationEnabled && sessionID) {
-          await managers.tmuxSessionManager.onSessionError({ sessionID });
-        }
-
         const errorName = extractErrorName(error);
         const errorMessage = extractErrorMessage(error);
         const errorInfo = { name: errorName, message: errorMessage };