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 84.99%. Comparing base (4e84609) to head (17698a2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5577      +/-   ##
==========================================
- Coverage   86.34%   84.99%   -1.35%     
==========================================
  Files          60       60              
  Lines       18663    18663              
==========================================
- Hits        16114    15863     -251     
- Misses       2549     2800     +251     

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[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.

[guhetier]

Updated the script with a simplified logic (it gets annoying to track state otherwise):

• one warning after 46 days
• close after 60 days
• any activity (other than setting the label) will remove the stale label

[guhetier]

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

The new script won't close all stale PRs immediately, but it will set the stale label immediately (starting the 2 weeks countdown)