fix: clone workspace into repo-named subdirectory directly

Git clone with an explicit destination puts the working copy directly into
that directory; it does not create a repo-named subdirectory. The previous
change assumed the opposite, so workspaces ended up at
/data/working-copies/{workspace_id}/ with the repo contents mixed in,
causing a 500 when the expected repo-named subdirectory was missing.

- Build the target path as /data/working-copies/{workspace_id}/{repo_name}/
  and pass it directly to GitService.clone
- Remove stale directory detection and fallback logic that is no longer
  needed
- Keep diagnostic logging around git clone failures

Quality gates:
- pytest tests/unit: 219 passed
- mypy: clean on changed files

apps/api/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: apps/api
 
 ## role
-Self-hosted FastAPI backend API that manages projects, git repositories, and development tools via Docker instances.
+FastAPI backend API that manages projects, git repositories, and development tools through Docker containers with PostgreSQL persistence.
 ## parent
 index: apps/.pi-map.index.md
 map: apps/.pi-map.md