ci: set up Mergify auto-merge with merge queue

[trim21]

Summary

• Add .github/mergify.yml: merge queue (max_parallel_checks: 1, update_method: merge) and an auto-merge rule for PRs to master that are approved or labeled ci:auto-merge, gated on the aggregate check test-checks. Stale Renovate lockfile PRs (>24h) are held back instead of auto-merging.
• Add a test-checks aggregate job to .github/workflows/test.yaml (needs: [test], ubuntu-slim) so Mergify references one stable check name instead of every matrix instance.
• .github/renovate.json: label Renovate PRs with ci:auto-merge (preset only applies dependencies) and set rebaseWhen: conflicted so Renovate only touches PRs that actually need a rebase.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.14%. Comparing base (74852a7) to head (528ae01).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   57.06%   57.14%   +0.08%     
==========================================
  Files          48       48              
  Lines        2287     2287              
==========================================
+ Hits         1305     1307       +2     
+ Misses        761      760       -1     
+ Partials      221      220       -1     

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[mergify]

This pull request does not currently match the merge queue conditions, so it cannot be queued from here. The box comes back if it matches again.