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

Look at issue #39 to understand the context and the existing ToolCallStatus union duplication. Start by examining the extension code that handles ACP tool_call and tool_call_update events. Create a shared type for ToolCallStatus, then implement TOOL_CALL_START and TOOL_CALL_UPDATE message forwarding. Design the ToolCallCard component with status pills and collapsible sections for input, result, and output previews. Test by simulating tool calls in the extension.

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