ci: Support incremental and nightly mutation testing

[geeknoid]

Addresses #60

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (32e413b) to head (6e2903a).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##             main      #62     +/-   ##
=========================================
  Coverage   100.0%   100.0%             
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  Files          33       52     +19     
  Lines        3291     6364   +3073     
=========================================
+ Hits         3291     6364   +3073     

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[Copilot]

Pull request overview

This PR introduces a two-tiered mutation testing strategy: incremental testing on PRs and comprehensive nightly testing. The incremental approach on PRs tests only the diff to reduce CI time, while the new nightly workflow performs full mutation testing on a weekly schedule (or manually), but only when there have been commits in the last 24 hours.

• Adds a new nightly workflow that runs full mutation testing weekly (Mondays at 5:30 AM UTC) with a gatekeeper to skip if no recent commits
• Modifies the main workflow to run incremental mutation testing on PR diffs only, reducing testing time from potentially 90+ minutes to 45 minutes
• Both workflows use the same tooling versions (cargo-mutants 25.3.1, Rust 1.91, sccache v0.12.0) for consistency

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
.github/workflows/nightly.yml	New workflow file that adds scheduled full mutation testing with a gatekeeper job to skip when no recent commits exist
.github/workflows/main.yml	Modified mutation testing to be incremental by generating and testing only the PR diff, reducing execution time

[geeknoid]

@copilot

[Copilot]

@geeknoid I've opened a new pull request, #63, to work on those changes. Once the pull request is ready, I'll request review from you.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.