Release pipeline: CHANGELOG, version bump, tagged releases

[bryanmatthewsimonson]

Right now there's no release story. manifest.json is pinned at 0.1.0, CI builds a zip on every push, but nothing gets tagged or published.

Minimum viable release flow

1. CHANGELOG.md at repo root, Keep-a-Changelog-ish format.
2. A script (or just documented recipe) to bump the version field in manifest.json, commit, tag v<n>.<n>.<n>.
3. A GitHub Actions workflow triggered on tag push that:
   • runs the existing web-ext lint + build,
   • attaches the resulting zip to a GitHub Release,
   • optionally: writes the CHANGELOG section for that tag into the Release notes.

Out of scope for now

• Publishing to the Chrome Web Store or AMO. That's a separate decision (store policies, review cycles, paid dev accounts). Self-hosted install via GitHub Releases zip is fine for early users.