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/firecracker/isolation_probe.py

@@ -108,7 +108,7 @@ def verify_isolation(private_key: Path, guest_ip: str) -> None:
         die(f"ISOLATION FAILURE: the VM reached the host canary "
             f"{canary_ip}:{canary_port}. The egress boundary is not in "
             f"force — refusing to run the agent (fail-closed). Verify the "
-            f"nft table with: ./cli.py firecracker setup")
+            f"nft table with: ./cli.py backend setup --backend=firecracker")
     if result.returncode == 2:
         die("isolation probe inconclusive: the guest has no python3/bash/nc "
             "to run the connectivity test. Refusing to continue "