Option to disable starter agent/team seeding (Fizz/Honey/Bumble, Welcome Team) for managed or self-hosted relays

## Context

Self-hosted business relay; members join by owner-minted invite. We deploy our own always-on org agents (buzz-acp on server infrastructure), so the desktop's starter personas are noise for our users.

## Problem

Every member's desktop seeds the built-in personas (Fizz, Honey, Bumble) and the built-in **Welcome Team** on every launch:

- `merge_personas` / `merge_teams` re-add any missing built-in by id on load, and `validate_persona_deletion` / `validate_team_deletion` reject deleting built-ins — so they cannot be removed, only edited.
- During onboarding, the Welcome Team can provision real agent instances against the org relay (in our case six members were created, one with an admin role, before we noticed and removed them).
- The only per-machine workaround we found is editing `managed-agents.json` (`is_active: false` on the built-ins, which the merge preserves) and emptying the built-in team — but that must be repeated on every employee's machine and survives only as long as the store format does.

For a company relay this is confusing at best (new users see three demo agents that have nothing to do with the org) and surprising at worst (auto-provisioned members on a closed relay).

## Ask

Any of these would solve it, in rough preference order:

1. A policy/config switch (org-managed file or env) that suppresses seeding of built-in personas and teams entirely.
2. A relay-advertised capability (e.g. NIP-11 field) that tells clients "this community manages its own agents — do not seed starters."
3. Make built-ins deletable, with the merge respecting a tombstone instead of re-adding by id.

Happy to contribute a PR if maintainers indicate a preferred direction.