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>

claude_bottle/backend/smolmachines/bottle_plan.py

@@ -48,7 +48,13 @@ class SmolmachinesBottlePlan(BottlePlan):
     # (push to a registry first, or smolvm grows a docker-daemon
     # transport).
     machine_name: str
-    agent_from_path: Path
+    # Agent image ref (docker tag). `launch` runs the
+    # build → save → registry push → smolvm pack pipeline against
+    # this and feeds the resulting `.smolmachine` artifact to
+    # `machine_create --from`. The pipeline runs at launch time
+    # (not prepare time) so the docker build output doesn't garble
+    # the dashboard's preflight modal.
+    agent_image_ref: str
     # In-guest env vars (HTTPS_PROXY etc) — IP-literal URLs since
     # the guest has no DNS resolver inside the TSI allowlist.
     # Passed to `smolvm machine create` as `-e K=V` flags.