feat(backend): default to smolmachines

bot_bottle/backend/__init__.py

@@ -25,7 +25,7 @@ backend exposes five methods:
       agents pane) to render a row.
 
 Selection is driven by `--backend` on `start` or
-BOT_BOTTLE_BACKEND (env var; default "docker"). Per PRD 0003 the
+BOT_BOTTLE_BACKEND (env var; default "smolmachines"). Per PRD 0003 the
 manifest does not carry a backend field; the host picks.
 """
 
@@ -550,11 +550,11 @@ def get_bottle_backend(
     `name` precedence:
       1. explicit arg (CLI `--backend=<name>` passes through here)
       2. BOT_BOTTLE_BACKEND env var
-      3. default `docker`
+      3. default `smolmachines`
 
     Dies with a pointer at the known backends if the chosen name
     isn't implemented."""
-    resolved = name or os.environ.get("BOT_BOTTLE_BACKEND") or "docker"
+    resolved = name or os.environ.get("BOT_BOTTLE_BACKEND") or "smolmachines"
     if resolved not in _BACKENDS:
         known = ", ".join(sorted(_BACKENDS))
         die(f"unknown backend {resolved!r}; known backends: {known}")

bot_bottle/backend/docker/backend.py

@@ -40,7 +40,7 @@ from .bottle_cleanup_plan import DockerBottleCleanupPlan
 from .bottle_plan import DockerBottlePlan
 class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanupPlan"]):
     """Docker backend implementation. Selected by BOT_BOTTLE_BACKEND
-    (default)."""
+    when set to `docker`; retained as a legacy/example backend."""
 
     name = "docker"
 

bot_bottle/backend/smolmachines/util.py

@@ -21,7 +21,9 @@ def smolmachines_preflight() -> None:
     die(
         "BOT_BOTTLE_BACKEND=smolmachines requires `smolvm` on "
         "PATH. Install with: "
-        "curl -sSL https://smolmachines.com/install.sh | sh"
+        "curl -sSL https://smolmachines.com/install.sh | sh. "
+        "To use the legacy Docker backend instead, set "
+        "BOT_BOTTLE_BACKEND=docker or pass --backend=docker."
     )