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/__init__.py

@@ -1,6 +1,6 @@
 """Main CLI dispatcher.
 
-Commands: cleanup, commit, edit, info, init, list, resume, start, supervise
+Commands: backend, cleanup, commit, edit, info, init, list, resume, start, supervise
 """
 
 from __future__ import annotations
@@ -13,10 +13,10 @@ from ..manifest import ManifestError
 from ..store_manager import StoreManager
 from ._common import PROG
 from . import list as _list_mod
+from .backend import cmd_backend
 from .cleanup import cmd_cleanup
 from .commit import cmd_commit
 from .edit import cmd_edit
-from .firecracker import cmd_firecracker
 from .info import cmd_info
 from .init import cmd_init
 from .resume import cmd_resume
@@ -26,10 +26,10 @@ from .supervise import cmd_supervise
 cmd_list = _list_mod.cmd_list
 
 COMMANDS = {
+    "backend": cmd_backend,
     "cleanup": cmd_cleanup,
     "commit": cmd_commit,
     "edit": cmd_edit,
-    "firecracker": cmd_firecracker,
     "info": cmd_info,
     "init": cmd_init,
     "list": cmd_list,
@@ -42,10 +42,10 @@ COMMANDS = {
 def usage() -> None:
     sys.stderr.write(f"usage: {PROG} <command> [args...]\n\n")
     sys.stderr.write("Commands:\n")
+    sys.stderr.write("  backend   set up or check a backend's host prerequisites (setup|status)\n")
     sys.stderr.write("  cleanup   stop and remove all active bot-bottle containers\n")
     sys.stderr.write("  commit    snapshot a running bottle's container state to a Docker image\n")
     sys.stderr.write("  edit      open an agent in vim for editing\n")
-    sys.stderr.write("  firecracker  one-time network setup for the Firecracker backend\n")
     sys.stderr.write("  info      print env, skills, and prompt details for a named agent\n")
     sys.stderr.write("  init      interactively create a new agent and add it to bot-bottle.json\n")
     sys.stderr.write("  list      list available agents or active containers\n")