feat(llm): Claude provider native tool_use

[bug-ops]

Problem

Claude provider sends plain text and parses bash blocks. Anthropic API supports native tool_use with structured responses.

Solution

• Serialize ToolDef → Anthropic tool format in chat request
• Parse tool_use content blocks from response
• Return ChatResponse::ToolUse with id, name, input
• supports_tool_use() returns true

Acceptance Criteria

• Tool definitions serialized in Claude API requests
• tool_use blocks parsed from responses
• Tool results sent back as tool_result messages
• Fallback to text mode when no tools provided

Part of #253 (M20)