docs(agents): add 3-minute subagent polling guidance

[shunk031]

Summary

• update the shared and Codex AGENTS guidance to require status polling within 180 seconds for gh_workflow_manager and worklog_manager
• clarify milestone updates for both subagents and how the parent agent should follow up with send_input when progress is unclear
• add a guidance test that covers the new 3-minute polling instructions

Testing

• not run locally (bats is validated in GitHub Actions per repo policy)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.79%. Comparing base (d68f030) to head (1dc47f0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #443      +/-   ##
==========================================
- Coverage   89.69%   88.79%   -0.91%     
==========================================
  Files           9        9              
  Lines         233      232       -1     
==========================================
- Hits          209      206       -3     
- Misses         24       26       +2     

Flag	Coverage Δ
macos-14-client	41.66% <ø> (ø)
ubuntu-latest-client	87.87% <ø> (-0.44%)	⬇️
ubuntu-latest-server	87.50% <ø> (-2.20%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.