fix: use login shell when dropping privileges in manifest entrypoint

The generated entrypoint used a brittle  invocation that could
spawn a non-interactive bash and exit immediately, causing containers to
stop with exit code 0 right after startup.

- Switch to  so the
  container user gets a login shell and stdin/tty are preserved
- Update the unit test assertion for the new drop-privileges command

Quality gates:
- pytest tests/unit: 219 passed
- ruff: clean on changed files
- mypy: clean on changed files

apps/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: apps
 
 ## role
-Contains the main application entry points and executable modules for the project.
+Contains the main deployable application modules that compose the project's executable entry points.
 ## parent
 index: ./.pi-map.index.md
 map: ./.pi-map.md