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/docker/backend.py

@@ -64,6 +64,11 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
         from . import setup as _setup
         return _setup.status()
 
+    @classmethod
+    def teardown(cls) -> int:
+        from . import setup as _setup
+        return _setup.teardown()
+
     def _preflight(self) -> None:
         _resolve_plan.preflight()
 

bot_bottle/backend/docker/setup.py

@@ -61,6 +61,15 @@ def setup() -> int:
     return 0
 
 
+def teardown() -> int:
+    sys.stderr.write(
+        "Docker backend: nothing to undo — it provisions no privileged host "
+        "state (networks and the sidecar bundle are per-launch and are "
+        "removed by `./cli.py cleanup`). Docker itself is left installed.\n"
+    )
+    return 0
+
+
 def status() -> int:
     ok = True
     if _docker_on_path():