feat(llm): extend LlmProvider trait with tool_use support

[bug-ops]

Problem

LlmProvider trait has no concept of tool definitions or structured tool calls. All tool interaction goes through text parsing.

Solution

Add to LlmProvider trait:

• fn supports_tool_use(&self) -> bool (default false)
• fn chat_with_tools(&self, messages, tools) -> Result<ChatResponse> (default delegates to chat())
• ChatResponse enum: Text(String) | ToolUse { id, name, input: Value } | Mixed(Vec<ContentBlock>)

Use schemars for JSON Schema generation from tool parameter types.

Acceptance Criteria

• Trait methods with default implementations (backward compatible)
• ChatResponse supports text + tool_use blocks
• JSON Schema generation via schemars derive
• Existing providers unaffected (return false for supports_tool_use)

Part of #253 (M20)