feat: plumb provider_id through message_created for replay

[morgmart]

Summary

• Adds provider_id to the acp:message_created event payload so assistant messages carry their provider identity during both live streaming and session replay
• During replay, the dispatcher reads the route-level provider_id (passed from the frontend's stored session metadata via load_session)
• The frontend's useAcpStream reads providerId from the event payload and stamps it on message metadata, enabling correct provider icon/name display for historical messages

Depends on

Requires block/goose PR — morganm/per-message-provider-id branch that persists provider_id in MessageMetadata on each assistant message.

Files changed

Tauri (Rust):

• payloads.rs — add provider_id field to MessageCreatedPayload
• writer.rs — include provider_id in live streaming event
• dispatcher.rs — include route provider_id in replay event
• session_ops.rs / manager.rs / command_dispatch.rs — thread provider_id through load_session
• commands/acp.rs — accept provider_id from frontend

Frontend (TypeScript):

• useAcpStream.ts — read providerId from event payload
• messages.ts — add providerId to MessageMetadata type
• acp.ts — pass providerId in acpLoadSession
• AppShell.tsx — pass session providerId when loading

Test plan

• Load a historical session — messages should show the session-level provider icon/name
• Send a new message — the live streaming path should stamp providerId from the event
• Switch providers mid-conversation and reload — messages get the thread-level provider (future improvement: per-message from backend)