Add workflow to close stale issues and PRs

[ethanwhite]

For us this means cases where we've asked the contributor to do something and they haven't responded for an extended period.

AI-Assisted Development

• I used AI tools (e.g., GitHub Copilot, ChatGPT, etc.) in developing this PR
• I understand all the code I'm submitting
• I have reviewed and validated all AI-generated code

AI tools used (if applicable):
Agent-assisted. All code reviewed for correctness.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.67%. Comparing base (2b26da4) to head (f4983db).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1424   +/-   ##
=======================================
  Coverage   86.67%   86.67%           
=======================================
  Files          28       28           
  Lines        4069     4069           
=======================================
  Hits         3527     3527           
  Misses        542      542           

Flag	Coverage Δ
unittests	86.67% <ø> (ø)

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