feat(firecracker): pull the infra rootfs as a prebuilt artifact (PRD 0069 Stage 2)

Stage 2 of the docker-free Firecracker backend (#348): stop building the
fixed infra image on the launch host. The infra VM's rootfs is host- and
bottle-agnostic (authorized_keys + guest IP ride the kernel cmdline, not the
rootfs), so it's built once off-host and published as a versioned, ready-to-
boot ext4; the launch host downloads + verifies + boots it — no Docker, no
image tooling, just HTTP + gunzip.

- infra_artifact.py: version = content hash of the rootfs inputs (the shipped
  bot_bottle package + the three Dockerfiles + the init), so a launch host
  pulls the artifact matching its code and a content change can't silently
  boot a stale rootfs. Pull + sha256-verify (fail-closed) + gunzip from a
  Gitea generic package; base/owner/token configurable, default this Gitea.
- infra_vm.ensure_built/boot default to the pull path; BOT_BOTTLE_INFRA_BUILD=
  local keeps the docker build-from-source path for iterating on Dockerfiles.
- publish_infra.py: the off-host half — builds the images with Docker, mke2fs
  the rootfs (with buildah slack), gzips, and PUTs it to the generic package.

Rollout note: default=pull means a launch 404s until an artifact is published;
until the Gitea packages endpoint is enabled + an artifact published, use
BOT_BOTTLE_INFRA_BUILD=local. Freeze/migrate's remaining docker use is a
separate PR.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UoEZHDjv84ChoZbozQERhJ

bot_bottle/backend/firecracker/infra_vm.py

@@ -35,7 +35,7 @@ from typing import Generator
 from ...log import die, info
 from ..docker import util as docker_mod
 from ..docker.gateway_provision import GatewayProvisionError
-from . import firecracker_vm, netpool, util
+from . import firecracker_vm, infra_artifact, netpool, util
 
 # The single infra-VM image: gateway data plane + baked control-plane source
 # (Dockerfile.infra FROM the gateway image). Built from source by default;
@@ -109,10 +109,26 @@ class InfraVm:
 
 
 def ensure_built() -> None:
-    """Build the infra image from source (bootstrap via host docker). The
-    infra image `COPY --from`s the orchestrator image and is `FROM` the
-    gateway image, so both must exist first. A pull-from-registry mode
-    replaces this later."""
+    """Ensure the infra rootfs is available before boot.
+
+    Default (docker-free, PRD 0069 Stage 2): download + verify the prebuilt
+    rootfs artifact matching this code version (see `infra_artifact`); the
+    launch host needs no Docker. `BOT_BOTTLE_INFRA_BUILD=local` instead builds
+    the three fixed images from source with host Docker — the infra image
+    `COPY --from`s the orchestrator image and is `FROM` the gateway image, so
+    both must exist first — for iterating on the Dockerfiles."""
+    if infra_artifact.local_build_requested():
+        build_infra_images_with_docker()
+        return
+    infra_artifact.ensure_artifact_gz(
+        infra_artifact.infra_artifact_version(_infra_init()))
+
+
+def build_infra_images_with_docker() -> None:
+    """Build the three fixed images from source with host Docker: orchestrator,
+    gateway, then the combined infra image (`COPY --from` orchestrator, `FROM`
+    gateway). The launch host uses this only in `BOT_BOTTLE_INFRA_BUILD=local`
+    mode; `publish_infra` uses it off-host to produce the published artifact."""
     docker_mod.build_image(
         _ORCHESTRATOR_IMAGE, str(_REPO_ROOT), dockerfile="Dockerfile.orchestrator")
     docker_mod.build_image(
@@ -190,10 +206,15 @@ def boot() -> InfraVm:
         die(f"orchestrator link {slot.iface} not present.\n"
             f"    ./cli.py backend setup --backend=firecracker")
 
-    base = build_infra_rootfs_dir()
     run_dir = _infra_dir()
     rootfs = run_dir / "rootfs.ext4"
-    util.build_rootfs_ext4(base, rootfs, slack_mib=8192)
+    if infra_artifact.local_build_requested():
+        util.build_rootfs_ext4(build_infra_rootfs_dir(), rootfs, slack_mib=8192)
+    else:
+        # Prebuilt artifact already carries the buildah build slack; expand it
+        # to a fresh writable rootfs for this boot.
+        infra_artifact.materialize_ext4(
+            infra_artifact.infra_artifact_version(_infra_init()), rootfs)
     private_key, pubkey = _stable_keypair()
 
     info(f"booting infra VM on {slot.iface} (guest {slot.guest_ip})")