## Why

Repository creation already supports cloning from a remote URL, but the current UI only accepts a full URL. For the common fixed-provider case (`git.commumedia.org`), users should be able to enter `owner` and `repo` and have the app verify the repository exists before cloning. If the repository does not exist, the app should surface a clear error. Existing blank repository creation must remain available.

## What Changes

- Change the shared repository create dialog to support an SSH-only clone form with `owner` and `repo`
- Build the clone target as `git@git.commumedia.org:{owner}/{repo}.git`
- Preflight clone targets with `git ls-remote` before cloning
- Return a clear error when the repository is missing or inaccessible
- Keep the current full URL paste flow as an advanced fallback
- Keep blank repository creation as a fallback option

## Capabilities

### Modified Capabilities

- `git-repo`: Repository creation UX and clone validation reuse the existing create endpoint and clone path

## Impact

- Frontend: `repository-create-dialog.tsx`, `git-repositories.tsx`, `repositories-settings-tab.tsx`
- Backend: `git_repositories.py` create endpoint clone preflight
- Docs: repository creation guidance must reflect SSH-only owner/repo input
- Tests: add coverage for SSH repo existence checks and fallback URL behavior
