Document mutation-testing workflow contract tests

[leynos]

Summary

• Adds a ### Workflow contract tests subsection under the existing
  ## Mutation testing section in docs/developers-guide.md, documenting
  tests/workflow_contracts/mutation_testing_test.py.
• Permutation: B/companion (Rust workspace with a companion crate outside
  the root workspace, extra-crate-dirs: "wireframe_testing").
• Contract-test style: hard-pinned. The test asserts the workflow's
  uses: line equals EXPECTED_USES, built from a PINNED_SHA constant,
  so bumping the pin requires editing the workflow and the test constant
  together in the same change (Dependabot cannot bump it unattended).
• Documents the local run command, make test-workflow-contracts
  (uv run --with 'pytest>=8' --with 'pyyaml>=6' pytest tests/workflow_contracts -q), and the six checks the test performs: the
  pinned uses: reference, job permissions, workflow-level default
  permissions, concurrency, triggers, and the with: block
  (extra-crate-dirs, exclude-globs, extra-args).

Roadmap / execplan note

docs/execplans/adopt-shared-mutation-workflow.md (Stage C) already
tracked updating the developers-guide "Mutation testing" section describing
the caller itself, and that work has landed. This PR adds incremental
documentation of the contract test that was not separately itemised in
that plan; no roadmap or execplan item needs ticking for this change.

Docs lint

make markdownlint — Summary: 0 error(s) (log content shown above; ran
clean, no fixes required).

Files

• docs/developers-guide.md

Test plan

• make markdownlint passes
• N/A — docs-only change, no Rust/Python test suite run

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