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/tools/mod.rs

@@ -57,8 +57,7 @@ impl Tool {
     }
 }
 
-// Re-export API wire types used by the agent turn loop
-use super::api::ToolCall;
+
 
 /// A tool call in flight — metadata for TUI + JoinHandle for
 /// result collection and cancellation.