Build OpenArc's source adapter

OpenArc ships no code for talking to any post source. It reaches its source through one small seam you implement locally, for the source the user is authorized to use, on the user's own account/session. Your job is to write openarc_adapter.py in the repo root so it satisfies that seam. The user is responsible for complying with their source's terms of service — surface that, don't work around it.

The contract

openarc_adapter.py must expose create_source(config) returning an object with two methods (the PostSource seam in src/openarc/source.py):

def search(self, keyword: str, since: datetime) -> list[Post]: ...
def publish_reply(self, post_id: str, text: str) -> None: ...