feat(firecracker): split orchestrator and gateway into separate VMs (PRD 0070)

Now that #469 got the DB off the data plane, the Firecracker infra runs as
two microVMs instead of one — mirroring the docker/macos plane split:

  * orchestrator VM (ORCH_IFACE) — control plane + buildah image builds; sole
    DB opener; host-seeded signing key. No gateway daemons.
  * gateway VM (new GW_IFACE) — egress / git-http / supervise data plane;
    mitmproxy CA + a host-minted `gateway` JWT (never the key). Reaches the
    orchestrator only over the one nft forward rule its link allows.

Both boot the SAME shared infra rootfs; a `bb_role=` kernel-cmdline arg
selects which plane a VM's PID-1 init starts, so there is still one published
artifact. The gateway learns the orchestrator's address via `bb_orch=` on the
cmdline (no IP baked into the artifact).

Isolation is nearly free: agents were already nft-dropped except the DNAT'd
gateway ports, so re-pointing that single DNAT rule at the gateway VM
(`dnat to gw_guest`) severs every agent's L3 route to the control plane. The
only added nft is the second infra link's mirror block (masquerade egress +
forward accept, which subsumes gateway->orchestrator) in the shared shell
script and the NixOS module.

netpool gains GW_IFACE + gw_slot() (the /31 above the orch link);
firecracker_vm.boot gains extra_boot_args for the role cmdline; infra_vm
ensure_running() boots + adopts the pair (orchestrator first, then the gateway
that resolves policy against it) and returns an InfraEndpoint mirroring the
docker/macos shape. Builds stay in the orchestrator (PRD 0070 v1); the gateway
is the slim unit.

Unit-tested (test_firecracker_infra_vm rewritten for two VMs; gw_slot helper
test added); the KVM boot / L3-isolation checks are validated on a Firecracker
host.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/firecracker/launch.py

@@ -11,15 +11,16 @@ Per bottle:
   6. provision (shared gateway CA, prompt, skills, workspace, git, supervise)
      over SSH.
 
-The per-bottle Docker sidecar bundle is gone. The shared gateway handles
-egress / git-gate / supervise for every VM; Docker's PREROUTING DNAT routes
-the VMs' traffic to it, and the nft table's `ct status dnat accept` rule
-in the forward chain lets it pass. The VM still sends to `host_tap_ip:PORT`
-— the address its world is, by nft design, limited to.
+The per-bottle Docker sidecar bundle is gone. The shared gateway (its own
+data-plane VM) handles egress / git-gate / supervise for every VM; the nft
+netpool's PREROUTING DNAT routes the VMs' gateway-port traffic to that gateway
+VM, and the nft table's `ct status dnat accept` rule in the forward chain lets
+it pass. The VM still sends to `host_tap_ip:PORT` — the address its world is,
+by nft design, limited to.
 
 Isolation is enforced by the operator-provisioned nft table (checked
-fail-closed in preflight): a VM reaches only the sidecar (DNAT'd from
-the host TAP IP) and nothing else.
+fail-closed in preflight): a VM reaches only the gateway (DNAT'd from the host
+TAP IP) and nothing else — not even the orchestrator control plane.
 """
 
 from __future__ import annotations
@@ -136,7 +137,7 @@ def launch(
         )
         # Point the agent's git-gate insteadOf rewrites and supervise MCP URL
         # at the shared gateway (reached at the slot's host TAP IP — the VM
-        # sends there and Docker DNAT routes to the gateway container).
+        # sends there and the nft netpool DNAT routes to the gateway VM).
         git_gate_url = (
             f"http://{slot.host_ip}:{_GIT_HTTP_PORT}" if git_gate_plan.upstreams else ""
         )