fix: resolve session race condition and follow-up turn output loss

Two bugs fixed:

1. agent-runner error path race condition: when agent process exited
   with error, the promise resolved immediately without waiting for
   outputChain, allowing a late wrappedOnOutput to re-persist a stale
   session ID after clearSession had already run.

2. follow-up turn state not resetting: when a follow-up IPC message
   joined an active run, turn-level flags (finalOutputSentToUser,
   sawNonProgressOutput) from the first turn prevented the last
   progress from being promoted to a final message.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/agent-runner.ts

@@ -681,10 +681,15 @@ export async function runAgentProcess(
           },
           'Agent exited with error',
         );
-        resolve({
-          status: 'error',
-          result: null,
-          error: `Agent exited with code ${code}: ${stderr.slice(-200)}`,
+        // Wait for any queued streamed-output handlers to finish so a late
+        // newSessionId cannot be persisted after the caller clears a poisoned
+        // session.
+        outputChain.then(() => {
+          resolve({
+            status: 'error',
+            result: null,
+            error: `Agent exited with code ${code}: ${stderr.slice(-200)}`,
+          });
         });
         return;
       }