Delete Memories by Caller

Delete every memory ever extracted for a specific caller on this agent, addressed by `caller_identity`. Privacy / GDPR surface. Rows become permanently unreachable immediately and are hard-deleted by the retention job after the tenant's configured retention window. Returns `200` with the soft-deleted row count rather than a bodiless `204`: the count is GDPR-purge audit evidence, so a privacy operator has direct proof of the purge without re-querying. This is a deliberate exception to the delete-returns-204 convention, shared with the caller-purge cascade (`DELETE /v1/agents/callers/{caller_id}`).