test: cover the blog/ frontmatter contract (tests/blog-frontmatter.test.mjs)

[kubestellar-hive]

Test Improvement

Adds tests/blog-frontmatter.test.mjs — the first test in this repository that reads
anything under blog/.

Docusaurus fails build outright when a post names an author key absent from
blog/authors.yml or a tag key absent from blog/tags.yml. npm run check (prettier,
markdownlint, cspell, markdown-link-check) never resolves frontmatter keys, so until now
a typo in an author key first appeared as a red deploy.

What it asserts, against the committed content

assertion	why it matters
every post authors: key resolves in blog/authors.yml	unresolved key is a fatal build error
every post tags: key resolves in blog/tags.yml	unresolved key is a fatal build error
post has title + slug, slug matches the filename after the YYYY-MM-DD- prefix	filename/slug drift silently changes the published URL
post slugs are unique	two posts at one URL
post has a {/* truncate */} (or <!-- truncate -->) marker	without it the blog index renders entire posts
each authors.yml entry has name, title, url, image_url, both URLs https	these are exactly the fields the author card renders
each tags.yml entry has label and description	rendered on the tag pages

Inline author objects (legal in Docusaurus) are skipped for key lookup rather than
rejected, so the test does not forbid a valid authoring style it does not need to check.

Verification

• node --test tests/blog-frontmatter.test.mjs — 8/8 pass at 00b44df.
• Mutation-checked, not just run green: pointing a post at a ghost-author key, at a
  nonexistent-tag key, and stripping a tags.yml description each fail the
  corresponding test with a message naming the offending file and key (3 failures, 5
  still passing). Working tree restored afterwards.
• Full suite npm run test:unit: 63/63 pass (55 before, +8).
• npx prettier --check clean.

Scope

One new file, tests/blog-frontmatter.test.mjs. No production code, no CI change, no
package.json or lockfile change — parsing uses the yaml ^2.9.0 devDependency the
repo already declares and already uses in scripts/collect-metrics.mjs and
scripts/import-architectures.mjs.

Disjoint from every open quality PR by construction: this is the only one that touches
blog/. It does not add or modify tests/helpers.mjs (#231), tests/helpers-jsx.mjs
or @swc/core (#229), tests/tools/ (#225), any scripts/ test (#185, #197, #208,
#214, #216, #221, #263), or any data/*.json contract test (#235, #239, #241, #253,
#257, #260).

Related Issue

Closes #269

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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.