Add GitHub Actions CI workflow for lint and tests (#79)

[stephaniehobson]

One-line summary

Run lint, the Vitest unit tests, and the Playwright end-to-end tests on every push to main and every pull request.

Significant changes and points to review

• Stop checking external links in the Playwright suite. loadsSuccessfully in tests/playwright/utils.js now skips any URL outside the app's origin, so internal links are still checked.
• Third-party bot walls failed these checks on GitHub's runners: donate.mozilla.org and x.com returned 403, and facebook.com blocks GitHub's runner IPs. External link rot, like Download Firefox links should link to firefox.com #448 fixed, is no longer caught automatically.
• Delete footer.spec.js and the contact page's "All links work" test, which checked only external and mailto: links.
• Bring AGENTS.md up to date with the Vite, Vitest, and Playwright toolchain, and say that CI now exists. CONTRIBUTING.md gets the same CI fix.
• Revise the ExecPlan skill (.claude/skills/execplans/SKILL.md and references/PLANS.md).
• Adds docs/execplans/2026-10-08-github-actions-ci-shorter.md.

Issue / Bugzilla link

#79

Testing

• Check that "CI / Lint and test" and "CI / End-to-end tests" both appear in this PR's Checks tab and pass.

[stephaniehobson]

@stevejalim This seems like A LOT for the amount of work that was actually done. Is this the kind of exec plan you expect to see or do I need to get it to be less verbose. (Do I really need to know it rebased?)

[stevejalim]

No, I don't think we need to knoiw it rebased and it's worth adding something like "don't log routine development operations in the execplan. Record consequential decisions and surprises" into https://github.com/mozilla/ensemble/blob/main/docs/execplans/README.md or - perhaps better - https://github.com/mozilla/ensemble/blob/main/.claude/skills/execplans/references/PLANS.md