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/web-ui/src/storage/stores/sessions-store.ts

@@ -1,4 +1,4 @@
-import type { AgentState } from "../../agent/agent.js";
+import type { AgentState } from "@mariozechner/pi-agent-core";
 import { Store } from "../store.js";
 import type { SessionData, SessionMetadata, StoreConfig } from "../types.js";