Document mutation-testing workflow contract tests

[leynos]

Summary

Adds a ## Mutation-testing workflow contract tests section to
docs/developers-guide.md, documenting the thin caller workflow at
.github/workflows/mutation-testing.yml
and the contract test that pins its shape.

Permutation: Python (mutmut), matching the shared brief's Permutation C,
reconciled to this repository's actual configuration:

• The caller's with: block carries exactly three inputs — paths,
  module-prefix-strip, and python-version — no exclude-globs or
  extra-args. Only these three are documented.
• The uses: pin follows the shape-only style: USES_RE in
  tests/test_mutation_workflow_contract.py asserts a full 40-character
  commit SHA without hard-coding the value, so Dependabot bumps it without a
  test edit.
• The contract test module has no skipif guard — it always executes
  and reads the workflow file directly from the checkout. This differs from
  the template's assumption of a mutmut-sandbox self-skip, so the section
  states the actual behaviour rather than the assumed one.
• Local run command, verified to pass (6 tests):
  uv run pytest tests/test_mutation_workflow_contract.py.

No table of contents exists in docs/developers-guide.md (the top of the
file is a list of accepted ADRs, not a section index), so no cross-link was
added.

No roadmap or execplan tracking applies: docs/roadmap.md and
docs/execplans/roadmap-normalization.md contain no mutation-testing task
to tick or annotate.

Docs lint

make markdownlint (markdownlint-cli2 + en-GB-oxendict spelling check via
typos): passes for the new section. One pre-existing MD049 emphasis-style
finding remains at line 862 (unrelated table caption, predates this change,
left untouched).

Test plan

• uv run pytest tests/test_mutation_workflow_contract.py — 6 passed
• make markdownlint — passes; no new findings from this change

[sourcery-ai]

Sorry @leynos, you have reached your weekly rate limit of 500000 diff characters.

Please try again later or upgrade to continue using Sourcery

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 062e8806-46f3-415d-85c2-84c522c771ef

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/mutation-contract-tests

---

_{Comment @coderabbitai help to get the list of available commands.}

[codescene-access]

No quality gates enabled for this code.