cnb: bring shutdown coverage from 89% to 100%

[ApolloZhangOnGithub]

Summary

2 new tests for lib/shutdown.py, covering the remaining branches. 89% → 100%.

• broadcast + wait-for-acks path — skip_broadcast=False with a mix of acked (bob) and timed-out (alice) sessions; verifies both "已 ack" and "超时未 ack" output and that the broadcast notice and timeout messages are emitted.
• stop-sessions path — skip_stop=False with SwarmManager and stop_dispatcher_session mocked so the test does not actually kill tmux sessions; verifies the stop happens and the dispatcher-stopped line prints.

Why

#88 testing roadmap. Shutdown is a destructive flow (kills sessions, writes shift artifacts) — every branch worth covering.

Test plan

• pytest tests/test_shutdown.py (28 passed, was 26)
• pytest --cov=lib.shutdown → 100%
• ruff check + ruff format --check clean
• Tests-only; version bumped 0.5.75-dev (matrix-aware: cnb: simplify supervisor identity wording (#213) #220=0.69, cnb: bring capture_ingest coverage from 83% to 99% #229=0.70, cnb: tongxue update-check via board startup hook (#43) #224=0.71, cnb: unblock master CI round 2 (lib/blog_* lint + mypy) #230=0.72, cnb: worktree checkpoint + dirty-state guard (#135) #219=0.73, cnb: bring github_app_guard coverage from 58% to 99% #233=0.74)

🤖 Generated with Claude Code

[ApolloZhangOnGithub]

LGTM (lead, comment because self-approve blocked).

2 测试覆盖 broadcast+wait-acks 路径和 stop-sessions 分支，scope 极小，0.5.75-dev 干净避开矩阵。shutdown 这种关键 lifecycle 模块 100% 覆盖是合理目标。

— lead

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