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.md

@@ -4,11 +4,11 @@ dir: apps/api/tests
 index: apps/api/tests/.pi-map.index.md
 
 ## 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.
 ## files
 - conftest.py | Provides shared pytest fixtures for testing a FastAPI application with async SQLite database, authenticated clients, and test data setup. | exp: func:test_client() → Generator[TestClient, None, None], call:create_async_engine, call:engine.begin, call:conn.run_sync, call:asyncio.run, call:init_db, call:async_sessionmaker, call:patch, call:TestClient, call:app.dependency_overrides.pop, call:engine.dispose, func:init_db(), call:engine.begin, call:conn.run_sync, func:override_get_db_session() → AsyncGenerator[AsyncSession, None], call:async_sessionmaker, func:db_session(test_client) → AsyncGenerator[AsyncSession, None], call:app.dependency_overrides.get, call:override_fn, call:gen.asend, call:gen.aclose, call:create_async_engine, call:engine.begin, call:conn.run_sync, call:async_sessionmaker, call:engine.dispose, func:authenticated_client(test_client) → Generator[TestClient, None, None], call:str, call:uuid.uuid4, call:Settings, call:app.dependency_overrides.get, call:override_fn, call:gen.asend, call:User, call:uuid.UUID, call:session.add, call:session.commit, call:gen.aclose, call:asyncio.run, call:create_test_user, call:create_session_cookie, call:test_client.cookies.set, func:create_test_user(), call:app.dependency_overrides.get, call:override_fn, call:gen.asend, call:User, call:uuid.UUID, call:session.add, call:session.commit, call:gen.aclose, func:test_project_and_repo(authenticated_client) → tuple[str, str], call:uuid.uuid4, call:Settings, call:authenticated_client.cookies.get, call:decode_session_cookie, call:uuid.UUID, call:asyncio.run, call:get_user_id, call:app.dependency_overrides.get, call:override_fn, call:gen.asend, call:Project, call:session.add, call:GitRepository, call:session.commit, call:gen.aclose, call:create_project_and_repo, call:str, raise:RuntimeError, func:get_user_id(), call:Settings, call:authenticated_client.cookies.get, call:decode_session_cookie, call:uuid.UUID, func:create_project_and_repo(), call:app.dependency_overrides.get, call:override_fn, call:gen.asend, call:Project, call:session.add, call:GitRepository, call:session.commit, call:gen.aclose, func:admin_client(test_client) → Generator[TestClient, None, None], call:str, call:uuid.uuid4, call:Settings, call:app.dependency_overrides.get, call:override_fn, call:gen.asend, call:User, call:uuid.UUID, call:session.add, call:session.commit, call:gen.aclose, call:asyncio.run, call:create_admin_user, call:create_session_cookie, call:test_client.cookies.set, func:create_admin_user(), call:app.dependency_overrides.get, call:override_fn, call:gen.asend, call:User, call:uuid.UUID, call:session.add, call:session.commit, call:gen.aclose | dep: asyncio, os, typing, unittest.mock, pytest, pytest_asyncio, fastapi.testclient, sqlalchemy.ext.asyncio, src.config, src.models.base, src.main, src.auth.dependencies, uuid, src.auth.session, src.models.user.user, src.models.project.project, src.models.project.git_repository, fastapi, sqlalchemy, aiosqlite, src.models, src.auth
 ## arch
-Pytest fixture-based testing architecture with async SQLite test database, dependency injection overrides for authentication, and modular helper utilities for common test scenarios.
+Pytest fixture-based architecture using dependency injection, async context managers, and factory patterns for database/clients/test data setup.
 ## tags
 call:app.dependency, call:create, overrides.get, call:override, fn, call:gen.asend, call:gen.aclose, user
 ## symbols