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/cleanup.py

@@ -0,0 +1,90 @@
+"""Cleanup for the Firecracker backend.
+
+Orphans are: firecracker VMM processes whose config lives under our run
+dir, the `bot-bottle-sidecars-*` containers, and the per-bottle run
+dirs. TAP slots free themselves (the flock drops when the launcher
+exits), so there is nothing to reclaim there.
+"""
+
+from __future__ import annotations
+
+import os
+import shutil
+import signal
+import subprocess
+from pathlib import Path
+
+from ...log import info
+from . import util
+from .bottle_cleanup_plan import FirecrackerBottleCleanupPlan
+
+_SIDECAR_PREFIX = "bot-bottle-sidecars-"
+
+
+def _run_root() -> Path:
+    return util.cache_dir() / "run"
+
+
+def _orphan_vm_pids() -> list[int]:
+    """firecracker processes whose --config-file is under our run dir."""
+    run_root = str(_run_root())
+    result = subprocess.run(
+        ["pgrep", "-a", "firecracker"],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        return []
+    pids: list[int] = []
+    for line in result.stdout.splitlines():
+        parts = line.split(None, 1)
+        if len(parts) != 2 or run_root not in parts[1]:
+            continue
+        try:
+            pids.append(int(parts[0]))
+        except ValueError:
+            continue
+    return pids
+
+
+def _sidecar_containers() -> list[str]:
+    result = subprocess.run(
+        ["docker", "ps", "-a", "--format", "{{.Names}}",
+         "--filter", f"name={_SIDECAR_PREFIX}"],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        return []
+    return sorted(n.strip() for n in result.stdout.splitlines() if n.strip())
+
+
+def _run_dirs() -> list[str]:
+    run_root = _run_root()
+    if not run_root.is_dir():
+        return []
+    return sorted(str(p) for p in run_root.iterdir() if p.is_dir())
+
+
+def prepare_cleanup() -> FirecrackerBottleCleanupPlan:
+    return FirecrackerBottleCleanupPlan(
+        vm_pids=tuple(_orphan_vm_pids()),
+        containers=tuple(_sidecar_containers()),
+        run_dirs=tuple(_run_dirs()),
+    )
+
+
+def cleanup(plan: FirecrackerBottleCleanupPlan) -> None:
+    for pid in plan.vm_pids:
+        info(f"kill firecracker VM pid {pid}")
+        try:
+            os.kill(pid, signal.SIGTERM)
+        except ProcessLookupError:
+            pass
+    for name in plan.containers:
+        info(f"docker rm -f {name}")
+        subprocess.run(
+            ["docker", "rm", "-f", name],
+            stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL, check=False,
+        )
+    for path in plan.run_dirs:
+        info(f"rm -rf {path}")
+        shutil.rmtree(path, ignore_errors=True)