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/alembic/versions/.pi-map.index.md

@@ -2,12 +2,14 @@
 dir: apps/api/alembic/versions
 
 ## role
-Database schema version control and incremental migration management for the API application, tracking the evolution of tables supporting users, SSH keys, projects, git repositories, tool types/instances, config profiles, workspaces, monitoring, notifications, and terminal sessions.
+Database schema version control and migration management for the API application, tracking incremental schema changes from initial tables through advanced features like workspaces, manifests, and monitoring.
 ## parent
 index: apps/api/alembic/.pi-map.index.md
 map: apps/api/alembic/.pi-map.md
 ## children
--
+- apps/api/alembic/versions/.ruff_cache
+  index: apps/api/alembic/versions/.ruff_cache/.pi-map.index.md
+  map: apps/api/alembic/versions/.ruff_cache/.pi-map.md
 ## files
 - 0001_initial_schema.py
 - 0002_refresh_tokens.py
@@ -50,6 +52,7 @@ map: apps/api/alembic/.pi-map.md
 - 2026_06_13_make_clone_mode_nullable.py
 - 2026_06_14_104415_add_tool_type_home_directory.py
 - 2026_06_14_182955_fix_pi_agent_home_directory_mount.py
+- 2026_06_15_090500_remove_pi_agent_explicit_repo_mount.py
 - 398082499c30_add_tool_config_fields.py
 - 6fc7bfcf199f_merge_remove_is_builtin_and_add_config_.py
 - 86cec91fdb00_merge_profile_resolver_and_workspaces_.py
@@ -68,5 +71,7 @@ map: apps/api/alembic/versions/.pi-map.md
   read: 2026_05_24_220141_add_startup_command.py, 2026_05_29_remove_lsio_command_override.py
 - change versions config
   read: 0003_user_configs.py, 0009_tool_configs.py, 0013_add_config_profiles.py
+- explore versions subdirectories
+  index: apps/api/alembic/versions/.ruff_cache/.pi-map.index.md
 ## dirty
 -