Attach url-shortener as git submodule (androidcast_project).

Standalone repo: git://f0xx.org/androidcast_project/url-shortener on branch main.
Removes vendored tree; adds backend/README.md and agent bootstrap note.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/README.md

@@ -0,0 +1,32 @@
+# Backend services (submodules)
+
+| Path | Remote | Role |
+|------|--------|------|
+| [url-shortener/](url-shortener/) | `git://f0xx.org/androidcast_project/url-shortener` | Standalone short-link API (`https://s.f0xx.org`) |
+
+## Clone / update
+
+```bash
+git submodule update --init backend/url-shortener
+```
+
+## Development flow
+
+1. **URL shortener changes** — work inside the submodule only:
+   ```bash
+   cd backend/url-shortener
+   git checkout main && git pull
+   # edit, test, commit, push to url-shortener remote
+   git push origin main
+   ```
+2. **Pin new submodule SHA in android_cast** — after the submodule push:
+   ```bash
+   cd ../..   # repo root
+   git add backend/url-shortener
+   git commit -m "Bump url-shortener submodule"
+   git push origin <your-branch>
+   ```
+
+Hub admin UI for bearer tokens lives in `examples/crash_reporter/backend/` (crashes console) — not in this submodule.
+
+Spec (monorepo): [docs/specs/20100611_3_url_shortener.md](../docs/specs/20100611_3_url_shortener.md)