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/src/services/.pi-map.md

@@ -4,11 +4,11 @@ dir: apps/api/src/services
 index: apps/api/src/services/.pi-map.index.md
 
 ## role
-Service layer for business logic encapsulation in the API application
+This directory serves as the services layer for the API application, intended to contain business logic and service implementations.
 ## files
 - __init__.py | Empty file with no functionality
 ## arch
-Minimal/placeholder package following Python package convention with empty __init__.py, awaiting future service module implementations
+The package follows a standard Python package structure with an empty `__init__.py` marker file, indicating it is a namespace package ready to house service modules, but currently contains no implemented services.
 ## tags
 init, empty, functionality
 ## symbols