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/cli/resume.py

@@ -52,8 +52,10 @@ def cmd_resume(argv: list[str]) -> int:
         user_cwd=metadata.cwd or USER_CWD,
         identity=metadata.identity,
     )
+    backend_name = metadata.backend or None
     return _launch_bottle(
         spec,
         dry_run=args.dry_run,
         remote_control=args.remote_control,
+        backend_name=backend_name,
     )