feat(backend): add teardown() to the contract; backend teardown

Add an abstract teardown() classmethod to BottleBackend — the inverse of
setup(), surfaced as `./cli.py backend teardown [--backend=NAME]`
(uninstall). Symmetric with setup: it prints the privileged commands /
declarative config change to remove the host prerequisites.

- firecracker: NixOS-aware — disable the flake module (or drop the
  import) and rebuild, or `firecracker-netpool.sh down` imperatively.
- docker / macos-container: nothing to undo (no privileged host state);
  print a short note.

Not called by the launch path or the test suite. Extends test_cli_backend
for the new dispatch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

bot_bottle/backend/macos_container/setup.py

@@ -46,6 +46,16 @@ def setup() -> int:
     return 0
 
 
+def teardown() -> int:
+    sys.stderr.write(
+        "macos-container backend: nothing to undo — it provisions no "
+        "privileged host state. The Apple Container CLI and its system "
+        "service are left as-is (stop the service yourself with "
+        "`container system stop` if you want).\n"
+    )
+    return 0
+
+
 def status() -> int:
     ok = True
     if _container.is_macos():