test: pin contributor template link form and fix the PR template's CONTRIBUTING link

[hivecommons-hive]

Test Improvement

.github/PULL_REQUEST_TEMPLATE.md is read in two places, and its only link
worked in just one of them. In the GitHub file viewer — where anyone editing
the template looks at it — [CONTRIBUTING.md](../CONTRIBUTING.md) resolves.
Where the template is actually read it does not: GitHub prefills it into a pull
request body, and a relative target there is resolved against the /compare
URL the contributor is on rather than against the repository, so the checklist
item every contributor sees points at a 404.

Nothing in the repository would have caught it:

• tests/issue-template-contract.test.mjs was the only thing reading the file,
  and it asserted presence and non-emptiness only.
• npm run check:links never sees it. _check:links-md in package.json is
  for f in *.md, which iterates root-level markdown and never descends into
  .github/.
• npm run check:markdown lints style, not resolution.

So this PR fixes the link and adds the guard that makes the next one fail at
npm run test:unit instead of reaching contributors.

What this adds

Five tests in tests/issue-template-contract.test.mjs, built on three helpers:

• markdownLinkTargets() — reads targets out of markdown, ignoring the title
  in [a](url "t") and skipping HTML comments, since a link parked in a
  comment is shown to nobody.
• badLinkTarget() — accepts an https:// URL or a #anchor, and explains
  why anything else is unusable in a rendered body.
• formLinkProblems() — applies that to everything GitHub renders out of an
  issue form: the form description, each markdown element's value, and
  each field's attributes.description.

Two decisions worth reviewing:

• Form, not liveness. The guard judges a target by its shape and never
  resolves it. Fetching targets is exactly what check:links is exempted from
  CI for in tests/workflow-scripts.test.mjs (GATES_NOT_RUN_BY_CI): a
  third-party outage must not fail an unrelated pull request. The same
  reasoning applies here.
• The guard is checked against the templates losing their links. An
  assertion that "no link is relative" passes just as well over a template with
  no links at all, so one test pins that the PR template still carries a
  CONTRIBUTING.md link for the others to judge.

The issue forms carry no relative links today
(grep -nE '\]\([^)h#][^)]*\)' .github/ISSUE_TEMPLATE/*.yml is empty), so for
them the guard is preventive; formLinkProblems is covered directly by a
synthetic form rather than by a defect that does not exist.

Files and functions claimed

• .github/PULL_REQUEST_TEMPLATE.md — the checklist's CONTRIBUTING.md link
  only.
• tests/issue-template-contract.test.mjs — new markdownLinkTargets,
  badLinkTarget, renderedFormText, formLinkProblems, and five tests. The
  existing tests are unchanged except that the PR template path moved into a
  shared PULL_REQUEST_TEMPLATE constant.

No production code, no workflow changes. Disjoint from every open PR: none of
#989, #991, #994, #996, #998, #1000, #1002, #1004, #1007, #1008, #1010, #1012,
#1014 or #1016 touches .github/PULL_REQUEST_TEMPLATE.md,
.github/ISSUE_TEMPLATE/** or tests/issue-template-contract.test.mjs
(verified with gh pr view <n> --json files over all fourteen).

Verification

• TZ=UTC node --test tests/issue-template-contract.test.mjs — 15/15 pass.
• The guard is red on the defect. With the template reverted to
  ../CONTRIBUTING.md, the pull request template links only with absolute URLs or anchors fails with
  ../CONTRIBUTING.md is relative; GitHub resolves it against the /compare URL when the template is rendered into a body, not against the repository.
• npm run test:unit:coverage:check — exits 0.
  src files 100.00% lines (8351/8351), 100.00% regions (2395/2395),
  unchanged. Repository-wide region coverage moves 95.02% → 95.04%.
• npx prettier --check — clean on both files.

Related Issue

Closes #1017

---

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.