Add contributing guide and CI

.github/workflows/test.yml

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+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+
+      - run: npm ci
+      - run: npx playwright install --with-deps chromium
+      - run: node --check github-sortout.user.js
+      - run: npm test

CONTRIBUTING.md

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+# Contributing
+
+Keep changes small, plain, and easy to audit.
+
+## Workflow
+
+Use pull requests for changes to `main`.
+
+1. Branch from `main`.
+2. Make the smallest change that solves the issue.
+3. Update docs when behavior, install steps, privacy, or testing changes.
+4. Open a pull request with what changed, why, and how it was tested.
+
+Avoid unrelated cleanup in feature or bugfix PRs.
+
+## Code
+
+- Keep the userscript dependency-free unless there is a strong reason.
+- Prefer DOM APIs and text nodes over `innerHTML`.
+- Do not add GitHub tokens, background services, analytics, or local activity tracking.
+- Treat GitHub selectors as unstable. Keep adapters small and covered by fixtures.
+- Sorting should only move loaded timeline activity, not the initial issue or PR post.
+
+## Testing
+
+Before opening a PR, run:
+
+```sh
+node --check github-sortout.user.js
+npm test
+```
+
+CI runs these same non-live checks on pull requests and pushes to `main`.
+
+Run the live smoke checks when touching GitHub page selectors or timeline behavior:
+
+```sh
+npm run test:live
+```
+
+Manual browser testing is still useful for userscript changes, especially in Firefox with Violentmonkey.
+
+## Releases
+
+There is no build step and no automatic update channel.
+
+For a release, update the userscript version, commit through a pull request, merge to `main`, and install from the raw script URL.