feat(firecracker): add Linux microVM backend to replace smolmachines

Adds a Firecracker-based backend for Linux, providing mature KVM-based
microVM isolation to replace smolmachines/libkrun (issue #342, closes
the dead-end tracked in #332).

Architecture:
- Guest control over SSH (dropbear injected into the rootfs) on a
  point-to-point TAP link. `ssh -t` forwards SIGWINCH natively, so no
  resize bridge is needed.
- Networking: a one-time, root-provisioned pool of user-owned TAP
  devices (no shared bridge → no docker0/virbr0/cni0 collisions) plus a
  dedicated `table inet bot_bottle_fc` nftables table (independent of
  Docker/ufw/firewalld rules). `./cli.py firecracker setup` prints the
  host-appropriate config (NixOS module or sudo script).
- Rootfs: `docker export` → ext4 via `mke2fs -d` (rootless, no mount),
  cached by image digest; per-bottle SSH pubkey + IP passed via the
  kernel cmdline.
- Sidecar: reuses the Docker bundle, published on the slot's host TAP IP.
- Fail-closed isolation: TAP pool verified at preflight; the egress
  boundary is proven empirically post-boot (before the agent runs) by a
  canary probe — the VM must fail to reach the host directly, or launch
  is refused.

Linux hosts with Firecracker + KVM now default to this backend;
macOS stays on macos-container.

Not yet validated end-to-end on live hardware (requires the one-time
network pool). Unit tests + pyright pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G8p32HJgPoS1hLPWubbftM

bot_bottle/backend/firecracker/freezer.py

@@ -0,0 +1,76 @@
+"""FirecrackerFreezer — snapshot a running microVM to a Docker image.
+
+The VM is live and can't be block-copied safely, so — like the macOS
+backend — we stream the guest root filesystem out over the control
+channel (SSH here) and rebuild an image from it. The bottle keeps
+running after the snapshot.
+"""
+
+from __future__ import annotations
+
+import json
+import os
+import subprocess
+import tempfile
+from pathlib import Path
+
+from ...log import die, info
+from .. import ActiveAgent
+from ..freeze import Freezer
+from . import util
+
+
+class FirecrackerFreezer(Freezer):
+    backend_name = "firecracker"
+
+    def _freeze(self, agent: ActiveAgent) -> str:
+        run_dir = util.cache_dir() / "run" / agent.slug
+        private_key = run_dir / "bottle_id_ed25519"
+        guest_ip = _guest_ip_from_config(run_dir / "config.json")
+        if not private_key.is_file() or not guest_ip:
+            die(f"cannot freeze {agent.slug}: run dir {run_dir} is missing the "
+                f"SSH key or VM config (is the bottle still running?)")
+        image_tag = f"bot-bottle-committed-{agent.slug}:latest"
+        _commit_via_ssh(private_key, guest_ip, image_tag)
+        info(f"committed {agent.slug} -> {image_tag!r}")
+        return image_tag
+
+    def _export_hint(self, slug: str, image_ref: str) -> None:
+        info(f"to export for migration:      docker image save {image_ref} "
+             f"-o {slug}.tar")
+
+
+def _guest_ip_from_config(config_path: Path) -> str:
+    try:
+        cfg = json.loads(config_path.read_text())
+    except (OSError, json.JSONDecodeError):
+        return ""
+    boot_args = cfg.get("boot-source", {}).get("boot_args", "")
+    for token in boot_args.split():
+        if token.startswith("ip="):
+            # ip=<client>::<gw>:<mask>::<dev>:off
+            return token[len("ip="):].split(":", 1)[0]
+    return ""
+
+
+def _commit_via_ssh(private_key: Path, guest_ip: str, image_tag: str) -> None:
+    with tempfile.TemporaryDirectory(prefix="bot-bottle-fc-commit.") as tmp:
+        rootfs_tar = os.path.join(tmp, "rootfs.tar")
+        ssh = util.ssh_base_argv(private_key, guest_ip)
+        with open(rootfs_tar, "wb") as tar_out:
+            result = subprocess.run(
+                [*ssh, "--", "tar", "--create", "--one-file-system",
+                 "--exclude=./proc", "--exclude=./sys", "--exclude=./dev",
+                 "--exclude=./run", "--file=-", "--directory=/", "."],
+                stdout=tar_out, stderr=subprocess.PIPE, check=False,
+            )
+        if result.returncode != 0:
+            die(f"ssh tar for {guest_ip} failed: "
+                f"{(result.stderr or b'').decode().strip() or '<no stderr>'}")
+        with open(os.path.join(tmp, "Dockerfile"), "w", encoding="utf-8") as f:
+            f.write("FROM scratch\nADD rootfs.tar /\nUSER node\nWORKDIR /home/node\n")
+        build = subprocess.run(
+            ["docker", "build", "-t", image_tag, tmp], check=False,
+        )
+        if build.returncode != 0:
+            die(f"docker build for {image_tag!r} failed")