fix: stage config-profile mounts as directories so ~/.config stays writable

- Switch apply_resolved_profile from per-file bind mounts to one
  directory-level bind mount per ResolvedMount target.
- Stage all configured files under instance_dir/mounts/<sanitized_target>
  and bind-mount that directory, so Docker no longer creates a root-owned
  parent directory such as ~/.config.
- Propagate read-only mode ('ro') as the 'readonly' flag on volume entries.
- Update unit tests to expect directory-level mounts and add coverage for
  readonly/writable flags.

Quality gates: python3 -m py_compile, pytest (313 passed, 34 skipped),
npm run typecheck, npm run lint.

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

@@ -2,7 +2,7 @@
 dir: apps/api/tests
 
 ## role
-Provides shared test infrastructure and fixtures for API integration testing.
+Provides shared test infrastructure and fixtures for the FastAPI API application.
 ## parent
 index: apps/api/.pi-map.index.md
 map: apps/api/.pi-map.md