Allow hiding or disabling built-in personas (Fizz/Honey/Bumble) and the Welcome Team

## Problem

The three built-in starter personas (Fizz, Honey, Bumble) and the built-in Welcome Team cannot be removed from the Agents panel:

- `validate_persona_deletion` hard-rejects built-ins with "Built-in agents cannot be deleted."
- `managed_agents/teams.rs` similarly rejects built-in team deletion ("Built-in teams cannot be deleted.")
- Even if the stored records are removed from `managed-agents.json` externally, the loader re-seeds missing built-ins on every app launch, so they always come back.

## Why this matters

For a self-hosted community running a curated roster of always-on agents (headless `buzz-acp` services on a server), the starter personas are permanent clutter in every device's Agents panel. They're dormant until someone clicks Start, but they:

- can't be distinguished at a glance from the "real" agents the team actually uses,
- invite accidental `Start` clicks that mint per-device agent copies (per-device duplicate identities are already a documented confusion source), and
- reappear after any cleanup, which reads as a bug to users who deleted them.

The starter personas are great for first-run onboarding — the request is only for a way to opt out after that.

## Suggested behavior (any of these would solve it)

1. A "Hide" action on built-in persona/team cards that persists as a local tombstone and survives the re-seed pass, or
2. a settings toggle ("Show starter personas") defaulting to on, or
3. respecting deletion of pristine built-ins (the seed loader already distinguishes pristine vs customized copies for retired seeds — the same mechanism could honor a user tombstone).

## Environment

- Buzz Desktop v0.5.8 (macOS, aarch64)
- Self-hosted relay (deploy/compose), closed membership
- Agents hosted headlessly via standalone `buzz-acp` launchd services

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start by tracing validate_persona_deletion, managed_agents/teams.rs, and the loader that reads managed-agents.json and re-seeds missing built-ins. Decide which suggested opt-out behavior fits the existing pristine-versus-customized seed handling. Done means a user can hide or disable the built-in personas and Welcome Team, the choice persists locally, and the entries do not return after relaunch or re-seeding.

Written by the indexing model from the issue text.