Phase 1: extract chat bubbles and tool renderers from SidebarChat

[Pterjudin]

Summary

• Extract ChatBubble, UserMessageComponent, and prose wrappers into sidebar-tsx/chat/
• Extract ToolHeader, ToolRenderers (builtin/MCP tools, streaming edit preview) into sidebar-tsx/tools/
• Shrink SidebarChat.tsx from ~3,912 → ~1,257 lines (~68% reduction)
• SidebarChat now owns composer shell: tabs, thread header, command bar, input, landing page

Module layout

sidebar-tsx/
├── SidebarChat.tsx          (~1,257 lines — shell only)
├── chat/
│   ├── ChatBubble.tsx       (messages, plan, review, checkpoint)
│   ├── UserMessageComponent.tsx
│   └── proseWrappers.tsx
└── tools/
    ├── ToolHeader.tsx
    └── ToolRenderers.tsx

Test plan

• npm run buildreact — build succeeds
• npm run test-phase0-qa — 88 unit tests passing
• Manual: send chat message — user/assistant bubbles render
• Manual: agent tool calls (read file, edit file) display correctly
• Manual: plan/review messages render with approve/reject flows
• Manual: landing page + composer input unchanged

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