feat: surface tool-call lifecycle as collapsible cards

Extracted from #39. Forward ACP `tool_call` / `tool_call_update` as TOOL_CALL_START and sparse TOOL_CALL_UPDATE messages (merged by toolCallId), with an extension-side helper that flattens tool content into short preview lines. Render via ToolCallCard: status pill (Pending/Running/Completed/Failed), inline input preview, expandable Input / Result preview / Output sections.

Cleanup while porting: the ToolCallStatus union is written out three times in #39; dedupe into one shared type.

**Size**: M | **Depends on**: #61 (content model); independent of #65 | **Tier**: 2