fix(agent): make Pi own queued message mutations

[0xSero]

Summary

• route queued Delete, Edit, and Steer through the Pi runtime instead of mutating React-only state
• use Pi's existing clearQueue, steer, followUp, and queue_update primitives to apply one exact queue mutation atomically
• preserve order and handle duplicate/context-wrapped prompts

Validation

• npm run check
• npm run test:integration
• 9 focused queue mutation regressions