feat(extension): browser extension for Chrome/Edge/Firefox + CI + v0.8.0

Adds a WXT-based browser extension that replaces manual JS snippet
injection for AI-driven browser testing. The extension auto-connects
to the Muyue server via WebSocket on every page, using the exact
same protocol as the existing snippet — zero backend changes needed.

- Chrome/Edge (MV3) + Firefox (MV2) from single codebase via WXT
- Content script: auto-connect WS, console capture, URL tracking, RPC
- Background service worker: token management, screenshots, badge
- Popup + side panel with server status, sessions, URL config
- CI workflows: build extension, attach .zip to releases
- Makefile targets: ext, ext-chrome, ext-firefox, ext-zip
- Version bumped to 0.8.0

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

.gitignore

@@ -32,3 +32,8 @@ vendor/
 
 # Frontend (web/.gitignore handles specifics)
 web/node_modules/
+
+# Extension build artifacts
+extension/node_modules/
+extension/.output/
+extension/.wxt/