feat: add Chrome extension browser relay

assets/chrome-extension/README.md

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+# Clawdbot Chrome Extension (Browser Relay)
+
+Purpose: attach Clawdbot to an existing Chrome tab so the Gateway can automate it (via the local CDP relay server).
+
+## Dev / load unpacked
+
+1. Build/run Clawdbot Gateway with browser control enabled.
+2. Ensure the relay server is reachable at `http://127.0.0.1:18792/` (default).
+3. Install the extension to a stable path:
+
+   ```bash
+   clawdbot browser extension install
+   clawdbot browser extension path
+   ```
+
+4. Chrome → `chrome://extensions` → enable “Developer mode”.
+5. “Load unpacked” → select the path printed above.
+6. Pin the extension. Click the icon on a tab to attach/detach.
+
+## Options
+
+- `Relay port`: defaults to `18792`.