[TASK][CORE-02.T06] Implement reference-safe retention and deletion

Parent: #3

Task ID: `CORE-02.T06`

Review: in progress
Implementation: in progress on branch `core-02-t06-retention`

Source: task breakdown from the implementation handbook and the parent issue implementation proposal.

Implementation PR: pending

Current scope: immediate retrieval suppression, scoped holds, resumable GC queue, reference-safe blob deletion.

Read the implementation handbook, parent issue #3, and the parent issue implementation proposal for CORE-02.T06; the current work is already in progress on branch core-02-t06-retention. Done means the listed scope—immediate retrieval suppression, scoped holds, a resumable GC queue, and reference-safe blob deletion—is implemented and reviewed.

Written by the indexing model from the issue text.