fix(smolmachines): build agent image in launch, not prepare

When starting a smolmachines agent from the dashboard the
docker-build output rendered on top of the curses preflight
modal — the build was kicked off before the operator had
confirmed launch. The docker backend's `prepare` is pure
resolution (no docker calls); smolmachines was inconsistent
because `prepare` called `_ensure_smolmachine` which ran
`docker build` → `docker save` → `crane push` → `smolvm pack
create`, several seconds of stderr noise rendered before the
y/N prompt.

Move the pipeline:

- `_ensure_smolmachine` (+ `_SMOLMACHINE_CACHE_DIR` + `_REPO_DIR`
  + the local-registry / smolvm imports) moves from
  `backend/smolmachines/prepare.py` to
  `backend/smolmachines/launch.py`. Called right before
  `_smolvm.machine_create` so the resulting `.smolmachine`
  sidecar path lands as a local in `launch`, not on the plan.

- `SmolmachinesBottlePlan.agent_from_path: Path` becomes
  `agent_image_ref: str`. `prepare` stashes only the docker tag
  (`$CLAUDE_BOTTLE_IMAGE` || `claude-bottle:latest`); `launch`
  resolves it into the artifact at bringup.

This puts smolmachines on the same prepare-vs-launch boundary
the docker backend uses: the preflight summary in the dashboard
prints, the operator confirms, then `launch` runs — and its
stderr is routed via `_route_op_to_right_pane` (in tmux) or via
`curses.endwin` (foreground handoff) so the build output lands
cleanly.

Tests:
- `tests/unit/test_smolmachines_prepare_image.py` →
  `tests/unit/test_smolmachines_launch_image.py`, updated to
  import `_ensure_smolmachine` from `launch` rather than
  `prepare`.
- `test_smolmachines_provision.py`: plan fixture switches
  `agent_from_path` → `agent_image_ref`.

593 unit tests pass.

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

tests/unit/test_smolmachines_launch_image.py

@@ -4,7 +4,12 @@
 Asserts that the cache-hit path returns without touching the
 registry / pack pipeline, and that the cache-miss path runs
 build → tag → push → pack in order against a registry port the
-helper yields."""
+helper yields.
+
+The pipeline lives in `launch.py` (moved from `prepare.py` so the
+docker build doesn't run before the dashboard's preflight modal;
+the curses-endwin / tmux pane-routing handoff happens around
+`launch`)."""
 
 from __future__ import annotations
 
@@ -13,14 +18,14 @@ import unittest
 from pathlib import Path
 from unittest.mock import patch
 
-from claude_bottle.backend.smolmachines import prepare as _prepare
+from claude_bottle.backend.smolmachines import launch as _launch_mod
 
 
 class TestEnsureSmolmachine(unittest.TestCase):
     def setUp(self):
         self._tmp = tempfile.TemporaryDirectory(prefix="cb-cache.")
         self._cache_patch = patch.object(
-            _prepare, "_SMOLMACHINE_CACHE_DIR", Path(self._tmp.name),
+            _launch_mod, "_SMOLMACHINE_CACHE_DIR", Path(self._tmp.name),
         )
         self._cache_patch.start()
 
@@ -35,20 +40,20 @@ class TestEnsureSmolmachine(unittest.TestCase):
         sidecar.write_text("")
 
         with patch.object(
-            _prepare.docker_mod, "build_image",
+            _launch_mod.docker_mod, "build_image",
         ) as build, patch.object(
-            _prepare.docker_mod, "image_id",
+            _launch_mod.docker_mod, "image_id",
             return_value=f"sha256:{digest}fffffffffffffffff",
         ), patch.object(
-            _prepare.docker_mod, "save",
+            _launch_mod.docker_mod, "save",
         ) as save, patch.object(
-            _prepare, "ephemeral_registry",
+            _launch_mod, "ephemeral_registry",
         ) as registry, patch.object(
-            _prepare, "crane_push_tarball",
+            _launch_mod, "crane_push_tarball",
         ) as push, patch.object(
-            _prepare._smolvm, "pack_create",
+            _launch_mod._smolvm, "pack_create",
         ) as pack:
-            result = _prepare._ensure_smolmachine("claude-bottle:latest")
+            result = _launch_mod._ensure_smolmachine("claude-bottle:latest")
 
         self.assertEqual(sidecar, result)
         # build still runs (Dockerfile edits land without manual rmi).
@@ -88,25 +93,25 @@ class TestEnsureSmolmachine(unittest.TestCase):
             return _f
 
         with patch.object(
-            _prepare.docker_mod, "build_image",
+            _launch_mod.docker_mod, "build_image",
             side_effect=record("build"),
         ), patch.object(
-            _prepare.docker_mod, "image_id",
+            _launch_mod.docker_mod, "image_id",
             return_value=f"sha256:{digest}fffffffffffffffff",
         ), patch.object(
-            _prepare.docker_mod, "save",
+            _launch_mod.docker_mod, "save",
             side_effect=record("save"),
         ) as save, patch.object(
-            _prepare, "ephemeral_registry",
+            _launch_mod, "ephemeral_registry",
             return_value=_Reg(),
         ), patch.object(
-            _prepare, "crane_push_tarball",
+            _launch_mod, "crane_push_tarball",
             side_effect=record("push"),
         ) as push, patch.object(
-            _prepare._smolvm, "pack_create",
+            _launch_mod._smolvm, "pack_create",
             side_effect=record("pack"),
         ) as pack:
-            _prepare._ensure_smolmachine("claude-bottle:latest")
+            _launch_mod._ensure_smolmachine("claude-bottle:latest")
 
         # Build → save → push → pack in that order. No `docker
         # push` (the daemon's HTTPS-by-default path is what we're

tests/unit/test_smolmachines_provision.py

@@ -90,7 +90,7 @@ def _plan(
         bundle_gateway="192.168.50.1",
         bundle_ip=bundle_ip,
         machine_name="claude-bottle-demo-abc12",
-        agent_from_path=Path("/tmp/agent.smolmachine"),
+        agent_image_ref="claude-bottle:latest",
         guest_env={},
         prompt_file=Path("/tmp/state/demo-abc12/agent/prompt.txt"),
         proxy_plan=PipelockProxyPlan(