No memwal_delete or supersede tool via MCP - stored memories cannot be corrected

Feature Request

Description

Once a memory is written via memwal_remember, there is no MCP tool to delete it, mark it as superseded, or replace it with a corrected version.

Problem

In production use, incorrect memories accumulate over time:

• A decision is reversed but the old record stays visible
• A bug is marked ''fixed'' but the original ''open'' record is never retired
• A wrong fact is stored and keeps surfacing in recall results

The only workaround is deduplication by similarity score, which is imprecise and cannot distinguish between ''same fact, updated'' vs ''genuinely different fact.'

Proposed Solution

Add a memwal_delete(blob_id, namespace) tool and/or a memwal_supersede(blob_id, replacement_text, namespace) tool that retires an old record and writes its replacement atomically.

The Continuity Keeper community prompt (Session 5) already works around this via a separate CLI (continuity supersede), but this functionality should be a first-class MCP tool.

Reported during Walrus Session 7 - Prompt Evolution

Start by tracing the existing memwal_remember MCP tool and reviewing the continuity supersede CLI mentioned in the issue. Compare the deletion and superseding behaviors, then define which MCP operation or operations are required and how atomic replacement should behave. Done means incorrect memories can be retired or replaced through MCP without relying on similarity deduplication.