Add stale issue management workflows

[ashwin-ant]

Summary

• Adds automated stale issue management to keep the issue tracker clean
• Issues get a warning comment after 30 days of inactivity
• Issues are automatically closed after 60 days total (30 days after warning)

Implementation

Two new workflows:

1. stale-issue-manager.yml - Runs nightly to check for stale issues

   • Adds warning comment and autoclose label to 30-day inactive issues
   • Closes issues that have been inactive for 60 days (where last comment was the warning)
   • Creates the autoclose label if it doesn't exist

2. remove-autoclose-label.yml - Handles user activity

   • Triggers when users comment on issues
   • Removes the autoclose label to reset the stale timer
   • Ignores bot comments to prevent false resets

Test plan

• Workflows pass YAML validation
• Test on a staging repo with old test issues
• Verify autoclose label gets created automatically
• Verify user comments remove the autoclose label

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