Strip api/types.rs to just Usage

Killed Message, Role, ToolCall, FunctionCall, MessageContent,
ContentPart, ImageUrl — all dead. types.rs is now 8 lines.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/api/mod.rs

@@ -10,8 +10,7 @@ pub mod http;
 mod types;
 mod openai;
 
-// Transitional — these will go away as callers migrate to AstNode
-pub use types::{Message, MessageContent, ContentPart, ImageUrl, Role, ToolCall, FunctionCall, Usage};
+pub use types::Usage;
 
 use anyhow::Result;
 use std::time::{Duration, Instant};