feat(drops): link release drops to forage items

What changed

This adds a first-class link from generated GitHub release drops to the GitHub issues or pull requests that grounded them:

• Added a drop_github_issues join table and schema associations between drops and cached GitHub forage items.
• Updated GitHub release drop sync to resolve referenced issues or pull requests, reuse cached forage records when available, and create missing records for connected repositories.
• Kept closed release-linked GitHub records in forage so the release history remains queryable after the normal open-issue sync runs.
• Added a reverse lookup for release drops linked to a GitHub forage item and included linked GitHub records in drop tool responses.
• Documented that release drops preserve the addressed forage-item relationship.

Why

Release drops currently pointed at source links, but Hive did not persist the graph between the shipped update and the work item it addressed. Persisting that relationship lets Hive answer product questions like which release addressed a given forage item.

Approach

The syncer now extracts GitHub references from generated release item source links. For each connected repository reference, it first checks the local forage cache, then fetches the missing GitHub issue or pull request only when needed. The drop-to-forage links are replaced idempotently on each sync so updates to release item grounding stay accurate.

Closed GitHub records created through release syncing are preserved when the open-issue reconciler deletes stale open issues. That keeps released work discoverable without changing the existing open-issue page behavior.

Impact

Operators get a small database migration for the new join table. Existing drops keep working, and new GitHub release drops start carrying linked forage items when their release item sources reference connected repositories. Tool clients can read the linked GitHub issue metadata from drop responses.

Validation

• mix format
• mix compile --warnings-as-errors
• mix test
• git diff --check

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