feat(desktop): add removable persistent audience chips

[BrianInAz]

Summary

• show named, removable persistent-agent audience chips above the Desktop composer
• removing a chip removes that agent from the current draft and the persisted thread audience
• preserve @mention as the path to add an agent back
• make persistent-mention reconciliation resilient to programmatic editor updates and duplicate display names

Related issue

None found. This builds on the persistent-audience behavior merged through #1949 and #1989.

Scope

This is intentionally independent of #4688. Current main already provides the persistent-audience storage, hydration, promotion, and send contracts this editor consumes. This PR neither imports nor includes #4688's contextual-conversation policy, ACP, Mobile, or settings changes.

Testing

• pnpm --dir desktop test
• pnpm --dir desktop build:e2e
• pnpm --dir desktop exec playwright test tests/e2e/persistent-agent-audience.spec.ts --project=smoke
• just ci
• mock-bridge E2E screenshot capture for the removable chips (posted in a follow-up comment)

Rebased and validated on block/buzz@feccf4eabc23fdba94ce3537a194357ed17b197c. The single commit carries DCO sign-off. No dependency, protocol, relay, or release-workflow changes are included.

[BrianInAz]

Removable persistent audience chips

The thread composer shows the two persisted agents by name; each chip has a dedicated remove control. The E2E test also verifies removal from the draft and stored audience, then @mention re-addition.