chore: promote the #124 docs into 5.3.1

[finalerock44]

Promote from dev to stable, carrying the documentation half of #124 that the 5.3.1 promotions dropped.

ci: stop reaching into the private dcd repo for the mock-api (#124) touched six files on dev. The promote PR (#123) carried four of them — cli-ci.yml, scripts/test-runner.mjs, the test:unit script in package.json, and eight of the ten changed CLAUDE.md lines — and dropped CONTRIBUTING.md and README.md entirely. Production therefore ships the new CI shape while its docs still describe the old one:

• CONTRIBUTING.md told contributors pnpm test boots a mock API, and that integration tests are "automatically skipped" on fork PRs only. Neither is true: CI runs no integration tests on any PR, and there is no default mock. It also omitted pnpm test:unit from the pre-push checklist, though it is now a required check.
• README.md's dev-scripts block still showed pnpm test # build + boot mock API + integration/unit tests.
• CLAUDE.md's Contributing section still described the DCD_SSH_DEPLOY_KEY mock-api checkout, which no longer exists.

README.md is the one with reach beyond this repo: npm always includes it in the tarball regardless of the files field, so the stale snippet would render on the npmjs.com page for 5.3.1.

Docs only — no source, workflow, script or lockfile change. The three files are now byte-identical to dev, leaving release plumbing (both manifests, CHANGELOG.md, package.json version) as the only remaining divergence, which is release-please's to own.

Release-As: 5.3.1 keeps the pending release PR (#120) on 5.3.1 rather than rolling it to 5.3.2.

Release-As: 5.3.1

What & why

Type of change

• fix — bug fix
• feat — new feature
• perf — performance improvement
• refactor — code change that's neither a fix nor a feature
• docs — documentation only
• chore / ci / build / test — tooling, no user-facing change
• Breaking change (title has ! or PR notes a BREAKING CHANGE:)

Checklist

• PR title follows the Conventional Commits format (see comment above)
• pnpm lint passes
• pnpm typecheck passes
• pnpm build passes
• I have not bumped the version or edited CHANGELOG.md (release-please handles this)
• I have signed the CLA (the bot will prompt on first contribution)
• Docs / README.md / STYLE_GUIDE.md updated if behaviour or output changed

How to test