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_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(