fix: E2E exit reason precedence + worktree prune race condition

Two fixes for E2E test reliability:

1. session-runner.ts: error_max_turns was misclassified as error_api
   because is_error flag was checked before subtype. Now known subtypes
   like error_max_turns are preserved even when is_error is set. The
   is_error override only applies when subtype=success (API failure).

2. worktree.ts: pruneStale() now skips worktrees < 1 hour old to avoid
   deleting worktrees from concurrent test runs still in progress.
   Previously any second test execution would kill the first's worktrees.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/worktree.test.ts

@@ -231,6 +231,9 @@ describe('WorktreeManager', () => {
     spawnSync('git', ['worktree', 'remove', '--force', oldPath], { cwd: repo, stdio: 'pipe' });
     // Recreate the directory to simulate orphaned state
     fs.mkdirSync(oldPath, { recursive: true });
+    // Backdate mtime to simulate a stale worktree (> 1 hour old)
+    const staleTime = new Date(Date.now() - 7200_000);
+    fs.utimesSync(oldRunDir, staleTime, staleTime);
 
     // New manager should prune the old run's directory
     const newMgr = new WorktreeManager(repo);