fix(do): isolate stop hook by task_id to prevent concurrent task interference

When running multiple do tasks concurrently in worktrees, the stop hook
would scan all do.*.local.md files and block exit for unrelated tasks.

Changes:
- setup-do.sh: export DO_TASK_ID for hook environment
- stop-hook.sh: filter state files by DO_TASK_ID when set, fallback to
  scanning all files for backward compatibility

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

skills/do/scripts/setup-do.sh

@@ -112,3 +112,4 @@ echo "Initialized: $state_file"
 echo "task_id: $task_id"
 echo "phase: 1/$max_phases ($phase_name)"
 echo "completion_promise: $completion_promise"
+echo "export DO_TASK_ID=$task_id"