Add markdown link check + PR template doc-update reminder to keep CONTRIBUTING/AGENTS from rotting

Once #407 lands the CONTRIBUTING.md + trimmed AGENTS.md, the documents will only stay accurate if drift is caught early. Two lightweight mechanisms would do most of the work:

Markdown link checker in pre-commit

Add the markdown-link-check (or lychee) hook to .pre-commit-config.yaml so every PR validates:

• Cross-file references (CONTRIBUTING.md -> LICENSE.txt, AGENTS.md -> CONTRIBUTING.md#before-you-push).
• Section anchors (catches when a section is renamed and a link to it stops resolving).
• External URLs (catches when the Slack invite, Zoom redirector, or upstream-tool links rot).

PR template reminder

Add one checklist item to .github/pull_request_template.md:

- [ ] If this PR changes contributor workflow, I've updated CONTRIBUTING.md / AGENTS.md

Refs #407.