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/resolve_plan.py

@@ -0,0 +1,47 @@
+"""Prepare step for the Firecracker backend."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from ...agent_provider import AgentProvisionPlan
+from ...egress import EgressPlan
+from ...env import ResolvedEnv
+from ...git_gate import GitGatePlan
+from ...manifest import Manifest
+from ...supervise import SupervisePlan
+from .. import BottleSpec
+from . import util
+from .bottle_plan import FirecrackerBottlePlan
+
+
+def preflight() -> None:
+    util.require_firecracker()
+
+
+def build_guest_env(resolved_env: ResolvedEnv) -> dict[str, str]:
+    return dict(resolved_env.literals)
+
+
+def resolve_plan(
+    spec: BottleSpec,
+    manifest: Manifest,
+    slug: str,
+    resolved_env: ResolvedEnv,
+    agent_provision_plan: AgentProvisionPlan,
+    egress_plan: EgressPlan,
+    supervise_plan: SupervisePlan | None,
+    git_gate_plan: GitGatePlan,
+    stage_dir: Path,
+) -> FirecrackerBottlePlan:
+    return FirecrackerBottlePlan(
+        spec=spec,
+        manifest=manifest,
+        stage_dir=stage_dir,
+        slug=slug,
+        forwarded_env=dict(resolved_env.forwarded),
+        git_gate_plan=git_gate_plan,
+        egress_plan=egress_plan,
+        supervise_plan=supervise_plan,
+        agent_provision=agent_provision_plan,
+    )