ci: build and publish extension packages from GitHub Actions

- build Firefox XPI and Chrome ZIP on every push
- upload packages as downloadable workflow artifacts
- sign and submit Firefox releases to AMO
- upload and publish Chrome releases to Chrome Web Store
- attach signed packages to GitHub release cards
- document required publishing secrets and release workflow

AGENT.md

@@ -16,6 +16,7 @@ This repository contains the Firefox WebExtension **Proxy Switcher Fork By WT**.
 - `options.html`, `options.js`, `options.css` — profiles, domains, management actions, and Advanced Settings.
 - `icons/` — extension icon sizes referenced by `manifest.json`.
 - `Makefile` — build and cleanup commands.
+- `.github/workflows/release.yml` — per-push package builds, workflow artifacts, and release-triggered AMO/Chrome Web Store publishing.
 
 ## Development rules
 
@@ -28,6 +29,7 @@ This repository contains the Firefox WebExtension **Proxy Switcher Fork By WT**.
 - When changing an icon, update every size referenced by the manifest and verify the XPI contains the new paths.
 - Never hand-edit or retain stale `META-INF` signatures after changing files. Build artifacts must be signed again before distribution.
 - Do not commit generated `web-ext-artifacts/` packages unless explicitly requested.
+- Keep publishing credentials only in GitHub Secrets. The release workflow expects AMO JWT credentials and Chrome Web Store OAuth credentials.
 
 ## Validation