History purge

Purpose: Remove retracted or operator-deleted prose from the repository's Git history on the server, deliberately and in batches, while keeping receipts and old commit-addressed links working. Scenario: An operator runs ./v3 purge-history preview, then run, confirming at the terminal or with --yes. The worklist is every post and signature path named by an accepted delete receipt. History is rewritten in place with built-in Git (fast-export through a path filter into fast-import), old refs are restorable until verification passes, then reflogs expire and the object database is pruned and checked by object id. A history rewrite record maps every changed commit identity and is committed with the purge; the read model and static pages are rebuilt. Sources: planner (source unavailable), rewrite primitive (source unavailable), engine and run (source unavailable), record spec, CLI (source unavailable), runbook. Known limitation: Backups, other clones and mirrors, downloads, provider copies and quotations are out of reach and are named in the output. A completed purge is undone only from the operator's backup.