refactor(backend): generic setup/status; drop firecracker-only CLI

Add abstract `setup()`/`status()` classmethods to BottleBackend (same
per-host, no-instance shape as is_available) so host provisioning is
part of the backend contract, not a per-backend command. Replace the
`./cli.py firecracker {setup,status}` command with a generic
`./cli.py backend {setup,status} [--backend=NAME]` that resolves a
backend (flag / $BOT_BOTTLE_BACKEND / host default) and dispatches —
swapping backends is just a different --backend.

Implementations:
- firecracker: moved out of cli/ into backend/firecracker/setup.py
  (network pool module/script + range-overlap check), unchanged output.
- docker: new backend/docker/setup.py — reports docker on PATH, daemon
  reachability, and gVisor runsc; setup notes no privileged pool is
  needed. Minimal placeholder; richer version tracked in #345.
- macos-container: new setup.py — container CLI + system-service checks.

Also retarget the launch-preflight / isolation-probe pointers to the new
command. New test_cli_backend covers dispatch + docker setup/status.

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

bot_bottle/backend/__init__.py

@@ -538,6 +538,28 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
         doesn't fail when `cli.py list active` walks past
         firecracker."""
 
+    @classmethod
+    @abstractmethod
+    def setup(cls) -> int:
+        """Emit this backend's one-time host setup — privileged network
+        pool, daemon bring-up, install pointers, etc. — as
+        host-appropriate config or commands. Prints to stdout/stderr and
+        returns a shell exit code (0 = nothing to report / success). A
+        backend that needs no host setup prints a short note and returns
+        0. Invoked generically by `./cli.py backend setup [--backend=…]`
+        so operators can provision any backend without a
+        backend-specific command. Classmethod (like `is_available`) —
+        it's a host query, not per-bottle state."""
+
+    @classmethod
+    @abstractmethod
+    def status(cls) -> int:
+        """Report whether this backend's prerequisites are satisfied on
+        the host — binaries, daemon reachability, network pool, range
+        conflicts, etc. Prints a human-readable summary; returns 0 when
+        the backend is ready to launch and non-zero when something is
+        missing. Invoked by `./cli.py backend status [--backend=…]`."""
+
 
 # Import concrete backend classes AFTER the base types are defined, so
 # each backend module can pull BottleSpec / BottlePlan / BottleBackend