WIP: Major cleanup - move Attachment to consumers, simplify agent API

- Removed Attachment from agent package (now in web-ui/coding-agent)
- Agent.prompt now takes (text, images?: ImageContent[])
- Removed transports from web-ui (duplicate of agent package)
- Updated coding-agent to use local message types
- Updated mom package for new agent API

Remaining: Fix AgentInterface.ts to compose UserMessageWithAttachments

packages/coding-agent/src/modes/rpc/rpc-mode.ts

@@ -187,7 +187,7 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 				// Hook commands and file slash commands are handled in session.prompt()
 				session
 					.prompt(command.message, {
-						attachments: command.attachments,
+						images: command.images,
 					})
 					.catch((e) => output(error(id, "prompt", e.message)));
 				return success(id, "prompt");