fix: use repository name for workspace mount target

WORKSPACE_NAME was computed from os.path.basename(repo_path), so when a
workspace path ended in a directory like 'main', the container mount target
became /home/user/main instead of /home/user/{repo-name}.

- Use GitRepository.name for WORKSPACE_NAME/REPO_NAME in manifest and
  legacy dockerfile flows
- Add unit test verifying prepare_manifest_instance uses repo.name even
  when the workspace path basename differs

Quality gates:
- pytest tests/unit: 213 passed
- ruff: clean on changed files
- mypy: clean on changed files

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

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