Add PR-validation workflow (run build on PRs, no deploy)

## Context

Falloutdocs currently has one workflow: \`.github/workflows/deploy.yml\`, which runs on push to \`main\`, on the daily schedule, and on manual dispatch. **There is no workflow that runs on pull requests.** This means every PR to this repo merges blind — any breakage (broken links once \`onBrokenLinks: 'throw'\` lands, TypeScript errors, dependency resolution failures, etc.) is only caught after the merge to main, at which point the site falls behind until someone fixes forward.

## Why this matters more now

PR #2 (ChrisonSimtian/Fallout#192 follow-up) flips \`onBrokenLinks: 'warn'\` to \`'throw'\`. Once that lands, any future content change in [ChrisonSimtian/Fallout/docs](https://github.com/ChrisonSimtian/Fallout/tree/main/docs) that introduces a broken link will fail the Falloutdocs build, and \`docs.fallout.build\` will stop receiving updates until a fix lands. PR-validation catches these before they hit main.

## Done when

- [ ] New \`.github/workflows/ci.yml\` runs on \`pull_request\` targeting \`main\`.
- [ ] Workflow checks out Falloutdocs and \`ChrisonSimtian/Fallout@main\` (same two-checkout shape as \`deploy.yml\`).
- [ ] Runs \`npm ci\` and \`npm run build\`.
- [ ] Does **not** deploy to GitHub Pages.
- [ ] Runs on Node 22 to match \`deploy.yml\`.

## Approach hints

- Easiest: copy the \`build\` job from \`deploy.yml\` into a new \`ci.yml\` with \`on: pull_request: branches: [main]\` and drop the \`upload-pages-artifact\` / \`deploy\` steps.
- Or refactor: extract the build steps into a reusable workflow (\`workflow_call\`) and call it from both \`ci.yml\` (no deploy) and \`deploy.yml\` (with deploy). Probably overkill for a two-workflow repo.