Automated release pipeline

When publishing a new version of this extension, these manual steps need to be taken by a human (me!):

**Chrome Web Store**

- Log into dashboard
- Zip extension `dist` directory
- Upload new version
- Update metadata if needed, for e.g. new permissions or changelog
- Submit for review

**Firefox Addons**

- Log into dashboard
- Zip extension `dist` directory
- Zip extension source code (manually excluding all the stuff in the `.gitignore`)
- Upload new version & source code
- Write changelog and reviewer notes
- Submit for review

That's a lot of work! And, more importantly, it's work that can be done by robots. There should be a GitHub Actions workflow that performs these actions on a release:

- [ ] Automatically writes a changelog based on commit messages between last version
- [ ] Automatically packages and uploads to Chrome Web Store
- [ ] Automatically packages and uploads **both** distribution and source to Firefox Addons
- [ ] Uploads a `.crx` and `.xpi` as release artifacts for people that want it for some reason

Start by inspecting the repository's existing release configuration, the dist directory, and .gitignore. Determine how a GitHub Actions workflow could package the extension and source, generate a changelog, and authenticate with both store dashboards. Done means a release produces Chrome and Firefox uploads plus .crx and .xpi artifacts, with the listed metadata and review information handled automatically.

Written by the indexing model from the issue text.