refactor(firecracker): single infra VM builds too — buildah in one image (PR #354 review)

Addresses the review finding that buildah lived only in the orchestrator
image, so the persistent infra VM wasn't the builder — a separate throwaway
builder VM contended with it for the orchestrator TAP. Consolidate:

- **Rebase the gateway (and thus infra) on `python:3.12-slim` = Debian
  trixie**, pip-installing mitmproxy instead of `FROM mitmproxy/mitmproxy`
  (Debian bookworm). trixie ships buildah 1.39, which can build agent
  Dockerfiles that use heredocs; bookworm's 1.28 can't (`Unknown
  instruction: "{"`). CA path is unchanged (set via `--set confdir=`).
- **buildah lives only in `Dockerfile.infra`** now (removed from the
  orchestrator image, which is lean/stdlib-only again).
- **Shared orchestrator content**: `Dockerfile.orchestrator` is the single
  definition of the control-plane payload; the infra image `COPY --from`s
  it (same trixie base → clean copy, and future deps like iroh are added
  once). The docker backend runs the orchestrator image directly.
- **`image_builder` builds inside the infra VM** (which now has buildah)
  over SSH — no throwaway builder VM, so the `bborch0` contention is gone.
  `ensure_built` builds orchestrator + gateway before infra (FROM gateway,
  COPY --from orchestrator).

Verified on a KVM host: images build (buildah 1.39 in infra), the agent
image builds *inside* the infra VM (heredoc Dockerfile and all), the infra
VM stays healthy, the agent boots and `claude --version` = 2.1.172. The
rebased gateway still starts as a docker container and generates its CA
(docker backend unaffected).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WBMWTEtQdJ4W5UrWuLHCck

tests/unit/test_firecracker_infra_vm.py

@@ -95,11 +95,14 @@ class TestRegistryVolume(unittest.TestCase):
 
 
 class TestEnsureBuilt(unittest.TestCase):
-    def test_builds_gateway_then_infra(self):
+    def test_builds_deps_before_infra(self):
         with patch.object(infra_vm.docker_mod, "build_image") as build:
             infra_vm.ensure_built()
         tags = [c.args[0] for c in build.call_args_list]
-        self.assertEqual([infra_vm._GATEWAY_IMAGE, infra_vm._INFRA_IMAGE], tags)
+        # infra is FROM gateway and COPY --from orchestrator, so both first.
+        self.assertEqual(infra_vm._INFRA_IMAGE, tags[-1])
+        self.assertIn(infra_vm._ORCHESTRATOR_IMAGE, tags[:-1])
+        self.assertIn(infra_vm._GATEWAY_IMAGE, tags[:-1])
 
 
 class TestWaitForHealth(unittest.TestCase):