feat(llm): add ToolUse and ToolResult message parts

[bug-ops]

Problem

MessagePart enum lacks variants for native tool use/result flow.

Solution

Add to MessagePart:

• `ToolUse { id: String, name: String, input: serde_json::Value }`
• `ToolResult { tool_use_id: String, content: String, is_error: bool }`

Wire through flatten_parts() for LLM content generation and SQLite persistence.

Acceptance Criteria

• New MessagePart variants
• Serialization/deserialization roundtrip
• flatten_parts() handles new variants
• SQLite persistence via existing parts JSON column

Part of #253 (M20)