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`.

tests/unit/test_docker_provision_git_user.py

@@ -30,7 +30,7 @@ class _Provider(AgentProvider):
     @property
     def runtime(self) -> AgentProviderRuntime:
         return AgentProviderRuntime(
-            template="test", command="test", image="", dockerfile="",
+            template="test", command="test", image="",
             prompt_mode="append_file", bypass_args=(), resume_args=(),
             remote_control_args=(),
         )