Document mutation-testing workflow contract tests

[leynos]

Summary

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

• Explains that the caller delegates to
  leynos/shared-actions/.github/workflows/mutation-mutmut.yml and only
  carries declarative configuration (the mutation run itself is
  informational, never gating).
• Documents the two with: inputs the caller actually sets: paths: cmd_mox/ and module-prefix-strip: "" (CmdMox uses a flat package
  layout, not src/).
• Describes tests/test_workflow_contract.py: a shape-only USES_RE pin
  (asserts a 40-hex commit SHA, not a specific value, so Dependabot bumps
  need no test edit) and its pytestmark = pytest.mark.skipif(...) guard,
  which self-skips inside mutmut's sandbox where .github/ is absent.
• Documents the local run command, verified to pass:
  uv run pytest tests/test_workflow_contract.py -v (there is no
  make test-workflow-contracts target in this repository's Makefile).

No existing mutation-testing section was present in the developer guide, so
this lands as a new top-level ## section, placed after the existing
"Workflow pins and Dependabot" section (which documents the general
shape-only-pin convention this contract test follows).

The developer guide has no table of contents / index list of its own
sections, so no cross-link was added there. docs/contents.md links to the
whole developer guide file already and did not need a new entry.

No roadmap or execplan tracking applies to this change; docs/roadmap.md
and docs/execplans/ contain no mutation-testing-related tracked task.

Docs lint

• make markdownlint: 0 errors.
• make spelling (via make markdownlint): passes; the new section uses
  en-GB Oxford -ize spelling (summarizing, serializes).

Test plan

• make markdownlint passes.
• uv run pytest tests/test_workflow_contract.py -v passes locally (6
  tests), confirming the documented command is correct.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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

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