feat: surface assistant thinking/reasoning stream

Extracted from #39. Forward ACP `agent_thought_chunk` session updates to the webview as a new THINKING_DELTA message; accumulate ThinkingPart entries interleaved with answer text in `useChat`; render via a collapsible ThinkingBlock (live "Thinking:" while streaming, "Show reasoning" when done).

Reference implementation: `ThinkingBlock.tsx` and the extension/session-update branches in #39.

**Size**: S-M | **Depends on**: #61 (content model); benefits from #60 | **Tier**: 2