fix(smolmachines): use containerized crane to push, bypassing docker daemon's HTTPS preference

The previous fix (`host.docker.internal:<port>` for daemon-side
push) still failed:

  Get "https://host.docker.internal:53958/v2/":
    http: server gave HTTP response to HTTPS client

`host.docker.internal` is reachable from Docker Desktop's daemon
VM but isn't in the daemon's default insecure-registries CIDRs
(only `::1/128` and `127.0.0.0/8` are), so docker push tries
HTTPS, hits a plain-HTTP registry, and refuses. The daemon.json
fix (`"insecure-registries": ["host.docker.internal"]`) works
but is a one-time manual step in Docker Desktop's UI — not
something we can do for the user.

Sidestep the daemon push entirely:

  1. docker build (as before) — local layer cache makes
     no-change rebuilds cheap.
  2. docker save the image to a per-digest tarball alongside the
     cached `.smolmachine`.
  3. Start an ephemeral registry container on a per-session
     docker network, with `-p :5000` so the host can also reach
     it for the pack step.
  4. docker run a one-shot crane container on the SAME network,
     mount the tarball, `crane push --insecure /img.tar
     <registry-container>:5000/...`. Container DNS resolves the
     registry on the network; `--insecure` forces plain HTTP.
  5. `smolvm pack create --image localhost:<host port>/...` from
     the host. smolvm's bundled crane auto-falls-back to HTTP
     for localhost addresses, so no insecure-registries config
     is needed on that side.
  6. Tear down everything; reap the tarball (registries hold the
     same bytes, no need to keep both around).

Net effect: the docker daemon never does an HTTP/HTTPS-policy
decision on our behalf. `docker push` is gone from the prepare
path; `docker save`, `docker network create`, `docker run` (for
registry + crane) replace it.

Tested end-to-end on Docker Desktop / macOS: `_ensure_smolmachine
("claude-bottle:latest")` produces a 204MB
`.smolmachine.smolmachine` artifact.

Adds:
- backend/docker/util.py:save() — thin docker save wrapper.
- local_registry.crane_push_tarball() — one-shot crane run on
  the registry's network.
- CRANE_IMAGE constant pinned by digest
  (gcr.io/go-containerregistry/crane@sha256:0ae17ecb...).

Removes:
- backend/docker/util.py:tag() / push() — unused without daemon
  push.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/docker/util.py

@@ -166,18 +166,15 @@ def image_id(ref: str) -> str:
     return r.stdout.strip()
 
 
-def tag(src: str, dst: str) -> None:
-    """`docker tag SRC DST`. Idempotent. Used by smolmachines prepare
-    to retag the locally-built image into a localhost:<port>/... ref
-    that the ephemeral registry will accept."""
-    subprocess.run(["docker", "tag", src, dst], check=True)
-
-
-def push(ref: str) -> None:
-    """`docker push REF`. Used by smolmachines prepare to push the
-    agent image into the ephemeral local registry so smolvm's crane
-    backend can pull it."""
-    subprocess.run(["docker", "push", ref], check=True)
+def save(ref: str, output: str) -> None:
+    """`docker save REF -o OUTPUT`. Writes a tarball of the image
+    layers + manifest to the host path. Used by smolmachines
+    prepare to hand the agent image to a containerized crane that
+    pushes it to the ephemeral registry — bypassing the docker
+    daemon's `docker push` (which on Docker Desktop can't reach a
+    host-loopback registry and refuses plain-HTTP pushes to
+    non-loopback hosts)."""
+    subprocess.run(["docker", "save", ref, "-o", output], check=True)
 
 
 def _silent_run(cmd: Iterable[str]) -> int: