test: cover the docs/ markdown link and frontmatter contract (tests/docs-contract.test.mjs)

[kubestellar-hive]

Test Improvement

Adds tests/docs-contract.test.mjs, the first test in the repo that reads the
docs/ tree. Six assertions over every docs/**/*.md{,x}:

• every relative doc-to-doc markdown link resolves to a file on disk (with .md/.mdx
  extension resolution, matching how Docusaurus resolves them)
• every #fragment on such a link matches a heading slug in the target document
• every frontmatter fence that opens is terminated
• top-level frontmatter lines are parseable key: value pairs
• docs under an autogenerated sidebar directory (architectures, community, per
  sidebars.js) declare a title
• the docs/ tree is non-empty, so the other assertions cannot pass vacuously

Why this is not already covered

docusaurus.config.js:33 sets onBrokenMarkdownLinks: 'warn'. onBrokenLinks: 'throw'
on line 32 covers resolved route links, not relative markdown paths between files — so a
rename or a deleted heading only emits a warning and deploy-gh-pages.yml still publishes
the page with a dead link. grep -rln 'docs/' tests/ returned nothing before this change.

Verification

node --test at rev 00b44df: 61 tests pass, 0 fail (55 existing + 6 new).
Each new assertion was mutation-checked to confirm it actually bites — pointing a link at a
missing file, adding a #no-such-heading fragment, and removing a title from
docs/community/awards.md each turned the corresponding test red, and the working tree was
restored clean afterwards. npx prettier --check passes on the new file.

Scope / non-overlap

Claims exactly one file, tests/docs-contract.test.mjs, and one cluster: markdown-link and
frontmatter integrity within docs/. It deliberately does not assert route or slug
uniqueness, navbar wiring or sidebar reachability (#275), the data/architectures
catalog-to-docs contract (#260), blog/ frontmatter (#270), adr/ records (#282), or
/img static asset resolution (#289). No production code, no content, no workflow files
are touched.

Note: docs/skills/*.md are excluded from the title/description assertions on purpose.
Four of them carry no frontmatter at all and none is referenced by sidebars.js or the
navbar, yet they publish because they live under docs/. Whether those pages belong on the
public site is a content decision for a maintainer, recorded in #294 rather than forced by
a test.

Related Issue

Closes #294

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Filed by quality agent (hold-gated mode). Human review required.

— hive: agent=quality backend=copilot model=claude-opus-5

[kubestellar-hive]

Important

Held for human review by the hive's ACMM level gate.

This PR was opened by the "quality" agent while Hive policy required a human checkpoint for that agent. Non-outreach agents are held at ACMM L3–L5; the outreach agent is always held because it publishes project-facing communication.

Hive will automatically remove the hold label once current policy no longer requires a level hold for "quality". If this is an outreach PR, a human must review it and remove the label.