[UI] Restore Subagent Activity Streaming #22

Problem

Subagent activity streaming is not working in real-time. Users only see brief flashes of activity status.

Root Cause Analysis

How it's supposed to work:

• Task component displays subagent activity via sync.data.message[childSessionId]
• Events should update the sync store in real-time
• Activity extracted from message parts shows current tool/operation

What's broken:

1. Missing message.updated events for child sessions

   • Child sessions emit message.part.updated events (parts stored correctly)
   • Child sessions emit message.created events
   • BUT message.updated events are NOT being emitted when messages are updated
   • The sync context only adds messages to sync.data.message[sessionID] on message.updated event

2. Activity display relies on manual sync

   • Child messages only appear via explicit sync.session.sync() API call
   • No REAL-TIME streaming of child session activity
   • Events arrive but messages aren't added to the store

Key Files

• packages/opencode/src/cli/cmd/tui/routes/session/index.tsx - Task component (~line 1800)
• packages/opencode/src/cli/cmd/tui/context/sync.tsx - Sync context event handling
• packages/opencode/src/session/index.ts - updateMessage function (~line 400)

Recommended Fix

1. Ensure message.updated events are emitted for child sessions when messages are created/updated

   • Verify Session.updateMessage is called during child session processing
   • Ensure event emission happens on every message change

2. Alternative: Direct event subscription in Task component

   • Have Task component subscribe to SDK events for its child session
   • Update local state on message.part.updated events
   • Bypasses sync context dependency

Related

• [UI] Subagent status line flickers and causes layout jumping #36 UI flicker (fixed) - was a symptom of this issue

Priority

High - affects UX when running parallel agents