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/cli/backend.py

@@ -0,0 +1,41 @@
+"""`backend` CLI command — generic host setup/status across backends.
+
+`./cli.py backend setup  [--backend=NAME]` provisions (or points at how
+to provision) the chosen backend's one-time host prerequisites.
+`./cli.py backend status [--backend=NAME]` reports readiness.
+
+Both dispatch to the backend's `setup()` / `status()` classmethods, so
+there are no backend-specific commands — swapping backends is just a
+different `--backend` (or `$BOT_BOTTLE_BACKEND`, or the host default).
+"""
+
+from __future__ import annotations
+
+import argparse
+
+from ..backend import get_bottle_backend, known_backend_names
+from ._common import PROG
+
+
+def cmd_backend(args: list[str]) -> int:
+    parser = argparse.ArgumentParser(
+        prog=f"{PROG} backend",
+        description="Set up or check a backend's host prerequisites.",
+    )
+    parser.add_argument(
+        "action",
+        choices=("setup", "status"),
+        help="setup: provision/print host prerequisites; status: report readiness",
+    )
+    parser.add_argument(
+        "--backend",
+        choices=known_backend_names(),
+        default=None,
+        help="backend to target (default: $BOT_BOTTLE_BACKEND or the host default)",
+    )
+    ns = parser.parse_args(args)
+
+    backend = get_bottle_backend(ns.backend)
+    if ns.action == "setup":
+        return backend.setup()
+    return backend.status()