## Summary
- During AI streaming, the full messages array (potentially 500KB+ with
many messages) was serialized and sent through IPC on every text-delta
chunk, causing UI sluggishness
- Changed to send only the streaming message ID and updated content for
high-frequency streaming updates
- Full messages are still sent for initial loads and state changes
(compaction, lazy edits) to maintain correctness
- Updated all consumers (`useStreamChat`, `usePlanImplementation`,
`useResolveMergeConflictsWithAI`) to handle both full and incremental
chunk modes

## Test plan
- [x] TypeScript compilation passes
- [x] All 897 unit tests pass (39 files)
- [x] E2E tests pass (234 passed — 20 pre-existing failures unrelated to
this change, confirmed by running on clean main)
- [x] Specifically tested: `new_chat`, `chat_input`,
`local_agent_advanced`, `local_agent_code_search`, `local_agent_grep`,
`local_agent_search_replace`, `local_agent_list_files`,
`local_agent_summarize`, `local_agent_connection_retry`,
`local_agent_read_logs`, `local_agent_consent`,
`local_agent_persistent_todos`, `local_agent_todo_followup`,
`concurrent_chat`, `chat_history`

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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---------
Co-authored-by: Will Chen <willchen90@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>