AGENTS.md Maintainer Automation

Create a cron automation that keeps each configured repository's AGENTS.md - the file an agent reads first when it starts work there - matching what the repository actually is. It is created when missing, updated when the repository has moved on, and left alone when it is still accurate.

The automation script is deterministic: scheduling, the once-per-week claim, the clone, the branch, the commit, the push, the pull request, and the clone's removal are all handled in Python. The LLM is invoked only to read the repository and write the file.

A week is one unit of work per repository, so a cron that fires more often than intended, a retried run, or a restarted service cannot open the same pull request twice. A repository whose previous pull request from this automation is still open is skipped, because a second one would edit the same file and reviewing it would tell you nothing the first did not.