fix: remove explicit repo mount from pi-agent manifest and derive workspace name from remote URL

The pi-agent manifest still declared an explicit repo mount with
{{WORKSPACE_NAME}}, making the mount target dependent on tool config. The
instance service now synthesizes the repo mount, so the manifest no longer
needs the explicit mount.

- Add Alembic migration 2026_06_15_090500 to remove the source_type: repo
  mount from the built-in pi-agent manifest
- Add _get_repository_mount_name() helper to derive the workspace directory
  name from the repository remote URL (matching git clone behavior) and
  fall back to the user-provided repository name
- Use the helper for WORKSPACE_NAME/REPO_NAME in manifest, legacy dockerfile,
  and legacy compose template paths
- Update unit tests for the new migration and helper

Quality gates:
- pytest tests/unit: 218 passed
- ruff: clean on changed files
- mypy: clean on changed files
- alembic heads: single head

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

@@ -2,7 +2,7 @@
 dir: apps/api/tests
 
 ## role
-Provides shared testing infrastructure and fixtures for the API application's test suite.
+Provides shared testing infrastructure and reusable fixtures for the API test suite.
 ## parent
 index: apps/api/.pi-map.index.md
 map: apps/api/.pi-map.md