Delete User Confirmed Project Memory

Interactive counterpart of the user-confirmed PUT: an explicit Remove click in the PI editor (behind its own confirm dialog) IS the user's review decision, so it may delete protected user-authored entries directly. Mirrors the PUT's guards exactly: interactive user tokens only (no process tokens — agent-originated deletions still go through memory_proposals), the four structured PI slugs only, and the live-hash If-Match check still applies.