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/enumerate.py

@@ -0,0 +1,43 @@
+"""Active-agent enumeration for the Firecracker backend.
+
+The agent runs in a VM (no container to list), so a live bottle is
+identified by its running sidecar container `bot-bottle-sidecars-<slug>`
+— the same discovery-by-prefix the other backends use.
+"""
+
+from __future__ import annotations
+
+import subprocess
+
+from ...bottle_state import read_metadata
+from .. import ActiveAgent
+
+_SIDECAR_PREFIX = "bot-bottle-sidecars-"
+
+
+def enumerate_active() -> list[ActiveAgent]:
+    result = subprocess.run(
+        ["docker", "ps", "--format", "{{.Names}}",
+         "--filter", f"name={_SIDECAR_PREFIX}"],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        return []
+    out: list[ActiveAgent] = []
+    for name in sorted(n.strip() for n in result.stdout.splitlines() if n.strip()):
+        slug = name[len(_SIDECAR_PREFIX):]
+        metadata = read_metadata(slug)
+        if metadata is None or metadata.backend != "firecracker":
+            # Skip sidecars owned by another backend (docker/smolmachines
+            # share the container-name prefix).
+            continue
+        out.append(ActiveAgent(
+            backend_name="firecracker",
+            slug=slug,
+            agent_name=metadata.agent_name,
+            started_at=metadata.started_at,
+            services=(),
+            label=metadata.label,
+            color=metadata.color,
+        ))
+    return out