feat(pr): PR entity foundation — schema v109, store CRUD, predicates (U1)
Adds the unified PR entity: pull_requests + pull_request_thread_state
tables (schema v109, re-runnable migration with three partial unique
indexes and a compat helper), PrEntity/PrThreadState types, store CRUD
(create-or-reuse, update, per-thread outcome upsert), and core-owned
predicates (isPrBacked merge-target-scoped, unverified hard gate,
auto-merge readiness). Migrates legacy branch_groups PR fields into
unverified entities (R19). Fixes the misleading 'transactional migration'
docstring.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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