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>
Kandev Agent
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