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_image_builder.py

@@ -1,6 +1,6 @@
 """Unit tests for the docker-free Firecracker agent-image builder.
 
-The VM boot / SSH / buildah plumbing (`_build_in_vm`) is integration-tested
+The VM boot / SSH / buildah plumbing (`_build_in_infra`) is integration-tested
 on a KVM host; here we cover the cache decision, the boot-bit injection, and
 the smoke-test no-op — the logic that must hold without a VM.
 """
@@ -29,7 +29,7 @@ class TestBuildAgentRootfsDir(unittest.TestCase):
         base.mkdir(parents=True)
         (base / ".bb-ready").write_text("ok\n")
         with patch.object(image_builder.util, "cache_dir", return_value=self.cache), \
-             patch.object(image_builder, "_build_in_vm") as build:
+             patch.object(image_builder, "_build_in_infra") as build:
             out = image_builder.build_agent_rootfs_dir(
                 self.dockerfile, image_tag="t:latest")
         build.assert_not_called()
@@ -37,7 +37,7 @@ class TestBuildAgentRootfsDir(unittest.TestCase):
 
     def test_cache_miss_builds_injects_and_marks_ready(self):
         with patch.object(image_builder.util, "cache_dir", return_value=self.cache), \
-             patch.object(image_builder, "_build_in_vm") as build, \
+             patch.object(image_builder, "_build_in_infra") as build, \
              patch.object(image_builder.util, "inject_guest_boot") as inject:
             out = image_builder.build_agent_rootfs_dir(
                 self.dockerfile, image_tag="t:latest", smoke_test=("claude", "--version"))
@@ -59,7 +59,7 @@ class TestBuildAgentRootfsDir(unittest.TestCase):
 class TestSmokeTest(unittest.TestCase):
     def test_empty_argv_is_noop(self):
         with patch.object(image_builder, "_ssh") as ssh:
-            image_builder._smoke_test(Path("/k"), "10.0.0.1", ())
+            image_builder._smoke_test(Path("/k"), "10.0.0.1", "tag", ())
         ssh.assert_not_called()
 
     def test_failed_smoke_dies(self):
@@ -67,7 +67,7 @@ class TestSmokeTest(unittest.TestCase):
         result = subprocess.CompletedProcess([], 1, stdout="broken", stderr="")
         with patch.object(image_builder, "_ssh", return_value=result), \
              self.assertRaises(SystemExit):
-            image_builder._smoke_test(Path("/k"), "10.0.0.1", ("claude", "--version"))
+            image_builder._smoke_test(Path("/k"), "10.0.0.1", "tag", ("claude", "--version"))
 
 
 if __name__ == "__main__":