Document agentic contribution workflow

Document that this repository is primarily maintained through agentic development under human guidance, prefers issue contributions, requires pull requests to have a matching issue, and uses Repo Assist for scheduled and maintainer-directed tasks.

No target file or test is named; first locate the repository's contributor or development documentation and review its existing workflow guidance. Document the agentic maintenance model, issue-first and matching-PR requirements, and Repo Assist usage, then verify that all four points are clear and consistent with the repository.

Written by the indexing model from the issue text.