Desktop: Welcome Team is recreated after users remove Fizz, Honey, and Pollen

## Summary

Buzz Desktop recreates the built-in Welcome Team agents (Fizz, Honey, and Pollen) after a user explicitly removes them from **My Agents**. There is no persistent opt-out state, so the Welcome Team reconciliation path reactivates the built-in personas and recreates their managed-agent instances.

## Environment

- Buzz Desktop: `desktop-v0.5.20`
- Self-hosted relay: `v0.5.2`
- Platform: macOS
- Relay membership required: enabled

## Steps to reproduce

1. Complete Desktop onboarding against a self-hosted relay.
2. Remove Fizz, Honey, and Pollen from **My Agents**, or delete their managed-agent instances.
3. Restart Buzz Desktop or re-enter the Welcome channel/onboarding flow.
4. Observe the Welcome Team reconciliation run again.

## Actual behavior

- The built-in personas are forced active again.
- Missing managed-agent instances are recreated.
- Welcome channel bot membership is restored.
- The relay receives new revisions for the same kind `30177` managed-agent coordinates.

## Expected behavior

An explicit user removal should persist across restarts and Welcome channel reconciliation. Removed Welcome Team agents should not be recreated unless the user explicitly chooses to restore them.

## Source-level diagnosis

`desktop/src/features/onboarding/welcomeGuide.ts` calls `ensureWelcomeTeam()` / `provisionWelcomeTeam()`, which:

1. calls `ensureWelcomeTeamPersonasActive()`,
2. looks up the three starter personas and relay-scoped instances,
3. creates missing managed-agent instances,
4. updates runtime/access settings, and
5. restores Welcome channel membership.

The in-process promise map prevents concurrent duplicate provisioning, but it is not a persistent user opt-out.

The supported managed-agent deletion path correctly emits an owner-signed NIP-09 coordinate tombstone for `30177::` and an identity archive request. The problem is that Welcome reconciliation can subsequently provision the instances again.

## Suggested fix

- Persist a `welcome_team_opt_out=true` state when the user removes the built-in Welcome Team.
- Check the opt-out before `ensureWelcomeTeam()` performs persona activation, agent provisioning, membership reconciliation, or Welcome intro work.
- Add an explicit **Restore Welcome Team** action that clears the opt-out.
- Add a regression test covering: remove agents → restart Desktop → enter Welcome channel → no new kind `30177` create/update events.

## Additional note

This is a Desktop lifecycle issue rather than a relay database cleanup issue. Direct deletion of relay event rows is neither necessary nor desirable; the existing signed tombstone/archive flow should remain the removal mechanism.