feat: AgentSidebar full functionality — chat display, streaming, events

[I-am-nothing]

Summary

The AgentSidebar shell (header tabs + input) is merged (#10). This issue covers the remaining functionality to make it production-ready.

Scope

Chat display

• Message list component (user messages, agent responses)
• Streaming text animation (token-by-token rendering)
• Markdown rendering in agent responses
• Auto-scroll to bottom on new messages
• Loading/thinking indicator

Event callbacks

• onSend(message: string) — already implemented
• onMicClick() — voice input trigger
• onFileClick() / onAttachFile() — file attachment trigger
• onTabSwitch(tabId: string) — tab change callback
• onTabClose(tabId: string) — tab close callback
• onTabCreate() — new tab callback
• onHistoryDelete(tabId: string) — history deletion callback
• onCollapse() / onExpand() — sidebar toggle callback
• onClose() — already implemented

Props refactor

• Accept tabs as a prop (currently internal state for demo)
• Accept messages per tab as a prop
• Accept activeTabId as controlled prop
• Accept isStreaming flag for streaming animation state

Polish

• Code block syntax highlighting in responses
• Copy button on code blocks
• Error state display
• Empty state (no messages yet)