Document mutation-testing workflow contract tests

[leynos]

Summary

Adds a ## Mutation-testing workflow contract tests section to
docs/developers-guide.md, documenting the informational mutation-testing
caller workflow and the pull-request-time contract test that pins its shape.

• Explains the caller's two run modes (daily change-scoped schedule; manual
  whole-workspace dispatch) and the with: inputs it passes to
  leynos/shared-actions/.github/workflows/mutation-cargo.yml
  (paths: "src/,crates/", exclude-globs: "test-support/**,crates/test-utils/**", extra-args: "--workspace --all-features --test-workspace=true").
• Notes the uses: reference pins the shared workflow to a full 40-character
  commit SHA, and that the contract test asserts only the shape of that pin
  (USES_RE), not a hard-coded SHA value, so Dependabot bumps it without any
  accompanying test edit.
• Documents the local run command,
  make test-workflow-contracts (backed by
  uv run --with 'pytest>=8' --with 'pyyaml>=6' pytest tests/workflow_contracts -q), and lists what the test asserts: the uses:
  pin, the exact with: block, least-privilege job permissions with an empty
  workflow-level default, per-ref concurrency without cancellation, and the
  trigger shape (daily schedule plus a plain workflow_dispatch).

The developers' guide is a short, flat document with no table of contents, so
no cross-link index entry was needed.

No docs/roadmap.md or execplan entry relates to mutation testing or this
documentation task, so no roadmap/execplan tracking applies.

Docs lint

make markdownlint (which chains the spelling gate) passes clean after
fixing two -ise/-ize spellings (summarising → summarizing,
serialises → serializes) introduced by the new prose.

Files changed

• docs/developers-guide.md

Test plan

• make markdownlint passes (spelling gate + markdownlint-cli2)
• No Rust/Python test suite run — docs-only change, per instructions

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