sync: port diff-scoped mutation CI from mutago

[jonbaldie]

Summary

• mutate only pull-request changed lines while retaining full-tree checks on master
• skip full-tree mutation runs for non-code pushes to master
• fetch full history for reliable merge-base filtering and document the setup

Validation

• actionlint .github/workflows/mutation.yml
• ./githooks/pre-commit
• full local mutation gate: MSI 81.59%, covered-code MSI 87.18%
• docs-only diff gate: 0 mutants, exit 0 in 14.79 seconds
• manual smoke test against github.com/jonbaldie/go-mutesting/v2/example