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/tests/unit/test_alembic_migrations.py

@@ -67,3 +67,23 @@ def test_remove_pi_agent_repo_mount_migration_imports() -> None:
     assert module.down_revision == "2026_06_14_182955"
     assert callable(module.upgrade)
     assert callable(module.downgrade)
+
+
+@pytest.mark.unit
+def test_remove_pi_agent_workspace_symlink_migration_imports() -> None:
+    migration_path = Path(__file__).parent.parent.parent / (
+        "alembic/versions/2026_06_19_113000_remove_pi_agent_workspace_symlink.py"
+    )
+    assert migration_path.exists()
+
+    spec = importlib.util.spec_from_file_location(
+        "remove_workspace_symlink_migration", migration_path
+    )
+    assert spec is not None and spec.loader is not None
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+
+    assert module.revision == "2026_06_19_113000"
+    assert module.down_revision == "2026_06_15_090500"
+    assert callable(module.upgrade)
+    assert callable(module.downgrade)