fix(tool): stop defaulting manifest working_dir to /workspace

The /workspace compatibility symlink was removed from the manifest
compiler/entrypoint in 3e59a25. Tool definitions that still set
runtime.working_dir to /workspace therefore start in an empty directory
instead of /home/user/{repo_name}.

- manifest-editor.tsx: default working_dir to empty instead of /workspace;
  update startup-script placeholder to reference /home/alex/.
- Add Alembic migration 2026_06_19_113000 that clears the stale
  runtime.working_dir = /workspace from the built-in pi-agent manifest.
- Add migration import test.

Quality gates: pytest tests/api tests/services/test_terminal_manager_multi.py tests/unit (248 passed), ruff check (clean), npx tsc --noEmit (clean), eslint (clean).

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

@@ -2,7 +2,7 @@
 dir: apps/api
 
 ## role
-Backend API server for the Headquarter platform, providing self-hosted project management, git repository orchestration, and development tool integration.
+Backend API server for the Headquarter platform, providing self-hosted project management, git repository management, and development tool orchestration services.
 ## parent
 index: apps/.pi-map.index.md
 map: apps/.pi-map.md