Add monthly PR clustering with evolving Rig strategies (#583)

* Add monthly PR clustering Rig workflow

Generate fresh clustering strategies from durable repo-memory history and publish the top three themes with the complete generated Rig.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

* Run PR clustering workflow daily

Keep the trailing-month analysis window and manual trigger while scheduling runs every day at 09:00 UTC.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: pelikhan