Both docker and smolmachines resolve_plan.py duplicated: slug minting,
metadata writing, agent state dir setup, git gate / egress / supervise
preparation, env_vars merge, and manifest dockerfile path resolution.
These are now consolidated in bot_bottle/backend/resolve_common.py.
Each backend's resolve_plan retains only its own logic (container name
resolution + env-file for docker; subnet allocation + guest_env build
for smolmachines).