Prevent duplicate Welcome Team memberships

[fenner888]

What changed

• Reconciles the private Welcome channel to the current local Fizz, Honey, and Bumble instances.
• Removes only stale built-in Welcome Team memberships published by the same Buzz owner.
• Preserves current local instances, custom agents, agents managed by another owner, non-agent members, and uncertain or malformed records.
• Treats relay or identity lookup failures as non-destructive: provisioning continues without pruning.

Root cause

Welcome Team provisioning previously checked only the current app's local managed-agent store. A second device, app bundle, or reset could create a new local trio and add it without removing the prior trio from the same Welcome channel.

User impact

Opening the same community from another installation no longer accumulates duplicate Fizz, Honey, and Bumble memberships in the private Welcome channel. This changes channel membership only; it does not delete agent definitions, credentials, messages, or application data.

Validation

• pnpm exec biome check src/features/onboarding/welcomeGuide.ts src/features/onboarding/welcomeGuide.test.mjs
• pnpm typecheck
• Focused Welcome guide tests: 18 passed
• Full desktop unit suite: 3,523 passed
• git diff --check