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>

lib/worktree.ts

@@ -259,6 +259,11 @@ export class WorktreeManager {
 
         const entryPath = path.join(worktreeBase, entry);
         try {
+          // Skip recent worktrees (< 1 hour old) to avoid killing
+          // worktrees from concurrent test runs still in progress
+          const stat = fs.statSync(entryPath);
+          const ageMs = Date.now() - stat.mtimeMs;
+          if (ageMs < 3600_000) continue;
           fs.rmSync(entryPath, { recursive: true, force: true });
         } catch { /* non-fatal */ }
       }