feat: persist backend in BottleMetadata; use it in resume and dashboard reattach (PRD 0040)

BottleMetadata gains a backend field (default ""). Docker prepare writes
"docker"; smolmachines prepare writes "smolmachines". read_metadata
deserialises it with "" as the backward-compatible default.

resume now passes metadata.backend to _launch_bottle so a preserved
smolmachines bottle is resumed on the right backend without requiring
BOT_BOTTLE_BACKEND to be set manually.

_bottle_for_slug now reads metadata.backend and constructs a
SmolmachinesBottle for smolmachines slugs instead of always defaulting
to DockerBottle. No-metadata slugs still fall back to Docker.

Closes #137

bot_bottle/backend/docker/bottle_state.py

@@ -105,6 +105,10 @@ class BottleMetadata:
     # written before chunk 3 (resume / inspect should fall back to
     # deriving from identity in that case).
     compose_project: str = ""
+    # PRD 0040: backend name ("docker" or "smolmachines"). Empty string
+    # for state dirs written before PRD 0040; callers default to "docker"
+    # for backward compatibility.
+    backend: str = ""
 
 
 def metadata_path(identity: str) -> Path:
@@ -138,6 +142,7 @@ def read_metadata(identity: str) -> BottleMetadata | None:
         copy_cwd=bool(raw.get("copy_cwd", False)),
         started_at=str(raw.get("started_at", "")),
         compose_project=str(raw.get("compose_project", "")),
+        backend=str(raw.get("backend", "")),
     )
 
 

bot_bottle/backend/docker/prepare.py

@@ -79,6 +79,7 @@ def resolve_plan(
         copy_cwd=spec.copy_cwd,
         started_at=datetime.now(timezone.utc).isoformat(),
         compose_project=f"bot-bottle-{slug}",
+        backend="docker",
     ))
     # Clear any leftover preserve marker from a prior capability-block
     # so this fresh launch can be cleaned up at session-end unless