Add Opera cache cleaner extension baseline

opera-cache-cleaner-extension/CONTRIBUTING.md

@@ -0,0 +1,25 @@
+# Contributing
+
+## Development
+
+This extension has no package manager or dependency install step. Edit the plain JavaScript, HTML, CSS, and manifest files directly.
+
+Run the syntax checks before submitting changes:
+
+```bash
+node --check background.js
+node --check popup/popup.js
+```
+
+## Manual Testing
+
+1. Open `opera://extensions`.
+2. Enable developer mode.
+3. Load this folder as an unpacked extension.
+4. Open the popup and verify cache clearing, timer save, timer disable, and status display.
+
+## Security Expectations
+
+- Do not add host permissions unless required by a documented feature.
+- Do not add cookie, history, downloads, or tabs permissions without explicit review.
+- Do not add external network calls.