refactor: replace runtime.dockerfile with AgentProvider.dockerfile property

Drop the `dockerfile` field from `AgentProviderRuntime` and replace it
with a convention-based `dockerfile` property on `AgentProvider`: the
base class looks for a `Dockerfile` file next to the provider's own
`agent_provider.py` module (via `inspect.getfile`), returning its path
or None. Built-in providers inherit the default automatically; custom
user providers work the same way by dropping a Dockerfile next to their
plugin file; any provider needing a non-standard path can override.

All callers (`docker/prepare.py`, `smolmachines/prepare.py`,
`capability_apply.py`) now resolve the provider object once and call
`.dockerfile` directly instead of reading `runtime.dockerfile`.

bot_bottle/contrib/codex/agent_provider.py

@@ -50,7 +50,6 @@ _RUNTIME = AgentProviderRuntime(
     template="codex",
     command="codex",
     image="bot-bottle-codex:latest",
-    dockerfile=str(Path(__file__).resolve().parent / "Dockerfile"),
     prompt_mode="read_prompt_file",
     bypass_args=("--dangerously-bypass-approvals-and-sandbox",),
     resume_args=("resume", "--last"),