feat(web): support ssh owner repo clone flow

- Add SSH-only owner/repo clone path for git.commumedia.org
- Preflight remote repository existence with git ls-remote before cloning
- Keep advanced URL paste fallback and blank repository creation
- Add focused backend and frontend coverage plus docs updates

Quality gates: python -m py_compile, vitest run src/components/repositories-settings-tab.test.tsx, npm run typecheck

docs/features/repositories.md

@@ -12,7 +12,7 @@ Git repositories are managed within projects. You can create bare repositories f
 2. Click the **"New Repository"** button
 3. Fill in the form:
    - **Name**: Repository name (required)
-   - **Remote URL**: For cloning (optional)
+   - **Owner** and **Repository**: For SSH cloning from `git.commumedia.org`
    - **Mirror Clone**: Toggle for mirror clones
 4. Click **"Create Repository"**
 
@@ -25,12 +25,12 @@ Creates a new bare git repository. Use this for:
 
 #### Clone from Remote
 
-Enter a git URL to clone from:
-- `https://github.com/user/repo.git`
-- `git@github.com:user/repo.git`
-- `https://gitlab.com/user/repo.git`
+Enter the repository owner and name to clone from `git.commumedia.org` over SSH:
+- `owner`: `alice`
+- `repository`: `demo`
+- Resulting SSH URL: `git@git.commumedia.org:alice/demo.git`
 
-**Smart URL Parsing:** If you paste a browser URL (like `https://github.com/user/repo/tree/main`), the system will automatically suggest the correct git URL.
+**Advanced fallback:** If needed, you can still paste a full git URL and the system will suggest the correct clone URL.
 
 #### Mirror Clone