Add AGENTS.md with development environment setup instructions

AGENTS.md

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+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+This is a static-asset-only repository containing JavaScript localization files for the Moot forums/commenting platform. There is no package manager, no build system, and no automated test framework.
+
+### Running the application
+
+Serve the repository root with any static HTTP server:
+
+```
+python3 -m http.server 8000
+```
+
+Then open `http://localhost:8000/test.html?<lang_code>` (e.g. `?en`, `?fr`, `?de`) in a browser. The page loads the corresponding `<lang>.js` file and renders the Moot forum widget with that locale.
+
+### Important notes
+
+- Files **must** be served over HTTP (not `file://`) for avatars and auth pieces to work.
+- The test page depends on external CDNs (`cdn.moot.it` for Moot CSS/JS, Google CDN for jQuery). If these are unreachable the forum widget won't render, but the language file itself still loads.
+- Language files must be UTF-8 encoded. If characters render incorrectly, re-encode with `iconv -f ISO-8859-1 -t utf-8`.
+- There are no lint, build, or automated test commands. "Testing" is manual: visually verify translations render in the browser.