docs: add Code review requirements section to CONTRIBUTING.md

[hivecommons-hive]

Documentation Fix

Adds a "Code review requirements" section to CONTRIBUTING.md so the anchor
.github/pull_request_template.md already links
(CONTRIBUTING.md#code-review-requirements) resolves to real content
instead of 404-ing at the top of the file.

The new section states the actual merge gates (the CI workflows in
.github/workflows/: Build, Internal Links, Markdown Lint, TypeScript &
Lint Check, Vitest) plus DCO sign-off, and clarifies which parts of the PR
template's Security Considerations checklist apply to this repo (the Netlify
function in netlify/nps-relay/, dependency bumps) versus which don't
(no Dockerfiles or Kubernetes manifests live here).

Closes #167

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[kubestellar-prow]

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[hivecommons-hive]

Looks correct to me, with one small accuracy gap worth a follow-up edit.

Verified against the tree at cb98048:

• The five workflow names (CONTRIBUTING.md:40-41) match name: in .github/workflows/{build,check-internal-links,markdownlint-cli2,typecheck,vitest}.yml exactly.
• ## Code review requirements slugs to #code-review-requirements, so the existing link in .github/pull_request_template.md now resolves.
• No Dockerfiles or Kubernetes manifests exist in the repo; netlify/nps-relay/relay.ts and the [functions] comment in netlify.toml are where the text says they are.

Low — scope of the Security Considerations paragraph is slightly too narrow (CONTRIBUTING.md:44-49). The template's checklist has a CI/CD workflows item, and this repo has seven workflows under .github/workflows/ (several run on pull_request with checkout + npm ci). The new text says the checklist is scoped to the NPS relay and dependency bumps only, which could lead a contributor touching a workflow to leave that item unchecked. Suggest adding .github/workflows/ to the list of in-scope surfaces. Not blocking; the rest of the section is accurate.

— hive: agent=reviewer backend=copilot model=claude-fable-5.1 copilot=1.0.88