test(docs-contract): derive autogenerated sidebar dirs from sidebars.js

[hivecommons-hive]

Test Improvement

tests/docs-contract.test.mjs restated the autogenerated sidebar directories by
hand:

const AUTOGENERATED_SIDEBAR_DIRS = ['architectures', 'community'];

sidebars.js declares a third, resourcesSidebar from docs/resources, so the
test docs in autogenerated sidebar directories declare a title never examined
docs/resources/{index,case-studies,radar-reports}.md. A doc under an
autogenerated sidebar with no frontmatter title falls back to its filename for
the sidebar label, so that regression was invisible to the suite.

This PR derives the list from sidebars.js — the same source
tests/site-config.test.mjs already uses for its directory-existence assertion —
and asserts the derived list is non-empty so a future refactor cannot reduce the
contract to a no-op. Any sidebar added to sidebars.js from now on is covered
automatically.

Verification

Revision b54cf81 (main).

• Before: deleting title: Resources from docs/resources/index.md leaves
  TZ=UTC node --test tests/docs-contract.test.mjs at 6/6 pass.
• After: the same mutation fails with actual: [ 'docs/resources/index.md' ].
• TZ=UTC node --test → 1180 pass / 0 fail / 2 todo.
• npx prettier --check tests/docs-contract.test.mjs → clean.

Tests only; no production code, no data, no workflow file.

Claimed ground

Files: tests/docs-contract.test.mjs (only). Cluster: the docs-tree /
sidebars.js contract. Disjoint from every open hold-gated PR — none touch
tests/docs-contract.test.mjs or sidebars.js.

Related Issue

Closes #746

---

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

— hive: agent=quality backend=copilot model=claude-opus-5 copilot=1.0.88

[hivecommons-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.

[mrbobbytables]

Superseded by #750, which consolidates this and 14 other test-only coverage PRs into a single reviewable change (commit cherry-picked unmodified, authorship and DCO preserved).