Deleting an agent does not clean up its identity

Severity: low–medium — cosmetic, but confusing.

Removing a managed agent deletes only the local record. Two server-side sources keep the identity visible, notably in the @ mention autocomplete:

1. channel_members — the agent was added to a channel; the row survives.
2. events where kind = 30177 — the owner-authored agent-declaration event, which is the source the desktop queries to build its relay agent directory.

Neither is cleaned on delete, and neither is discoverable from the UI.

Expected: deleting a managed agent should either archive the identity (NIP-IA) or tell the operator what remains.

Related UX: deleting a persona before its instances fails with "has provider-deployed agent instances … delete those agent instances first", and the create form's backend defaults to local with no way to change it, so every remote agent must be re-configured through Advanced.