Add workflow to auto-close stale PRs after 60 days

[Copilot]

Description

Implements automated cleanup of inactive PRs to address the backlog of 69+ open PRs, many stale for months or years.

Workflow behavior:

• Day 46: First warning comment + stale label
• Day 53: Final warning comment + stale-final-warning label
• Day 60: PR auto-closed with notification
• Updated PRs have stale labels automatically removed

Exemptions:
PRs labeled pinned, security, or dependencies are never auto-closed.

Schedule:
Runs daily at 00:00 UTC via cron, manually triggerable via workflow_dispatch.

Implementation:
Uses actions/github-script@v7 for custom logic with full error handling and logging. Processes all open PRs via pagination.

Testing

No existing tests. Workflow syntax validated. Logic tested with mock data to verify correct behavior at each threshold (46/53/60 days).

Documentation

No documentation changes needed. Workflow is self-documenting via inline comments.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Stale PRs should be auto-closed</issue_title>
<issue_description>### Describe the feature you'd like supported

We have 69 active PRs into MsQuic, and many of them haven't been updated for months or years. We should clean those up and stay clean.

Proposed solution

Add a periodic action that scans for PRs not modified within 60 days and closes those PRs. The PR author should be notified 2 weeks and 1 week before it gets closed.

Additional context

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Stale PRs should be auto-closed #5576

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.57%. Comparing base (f5eddcc) to head (6433d8e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5577      +/-   ##
==========================================
- Coverage   85.85%   85.57%   -0.28%     
==========================================
  Files          59       59              
  Lines       18622    18622              
==========================================
- Hits        15988    15936      -52     
- Misses       2634     2686      +52     

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[mtfriesen]

I wonder if these comments will refresh the "last updated" date?

[anrossi]

Will this immediately close all currently stale PRs? Or will it start the clock from when it first runs?

[mtfriesen]

Will this immediately close all currently stale PRs? Or will it start the clock from when it first runs?

Yeah I also wondered about its first run vs periodically going forward. I think it might insta-close the current PRs, which are still old, and can still be reactivated if that were to occur and the author wants to bring it back.