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Phase 4 of multi-repository task support: github_task_prs and github_pr_watches gain a repository_id column so multi-repo tasks can have one PR per repo without colliding on (task_id) uniqueness. - Idempotent ALTER TABLE adds repository_id to both tables. - TaskPR.RepositoryID and PRWatch.RepositoryID model fields. - New GetTaskPRByRepository and ListTaskPRsByTask lookups; existing GetTaskPR keeps single-PR semantics for callers not yet migrated. - ReplaceTaskPR's delete is scoped by repository_id when set so a multi-repo task replacing one repo's PR no longer wipes the other. - Tests cover per-repo storage and scoped replace. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>