WIP: Refactor agent package - not compiling

- Renamed AppMessage to AgentMessage throughout
- New agent-loop.ts with AgentLoopContext, AgentLoopConfig
- Removed transport abstraction, Agent now takes streamFn directly
- Extracted streamProxy to proxy.ts utility
- Removed agent-loop from pi-ai (now in agent package)
- Updated consumers (coding-agent, mom) for AgentMessage rename
- Tests updated but some consumers still need migration

Known issues:
- AgentTool, AgentToolResult not exported from pi-ai
- Attachment not exported from pi-agent-core
- ProviderTransport removed but still referenced
- messageTransformer -> convertToLlm migration incomplete
- CustomMessages declaration merging not working properly

packages/agent/src/index.ts

@@ -1,22 +1,6 @@
 // Core Agent
-export { Agent, type AgentOptions } from "./agent.js";
-// Transports
-export {
-	type AgentRunConfig,
-	type AgentTransport,
-	AppTransport,
-	type AppTransportOptions,
-	ProviderTransport,
-	type ProviderTransportOptions,
-	type ProxyAssistantMessageEvent,
-} from "./transports/index.js";
+export * from "./agent.js";
+// Loop functions
+export * from "./agent-loop.js";
 // Types
-export type {
-	AgentEvent,
-	AgentState,
-	AppMessage,
-	Attachment,
-	CustomMessages,
-	ThinkingLevel,
-	UserMessageWithAttachments,
-} from "./types.js";
+export * from "./types.js";