Refactor session module structure and improve error handling

- Rename session.ts to index.ts for cleaner module imports
- Update all imports to use new session module structure
- Add error metadata tracking to message schema
- Improve error handling in session stream processing

🤖 Generated with [OpenCode](https://opencode.ai)

Co-Authored-By: OpenCode <noreply@opencode.ai>

packages/opencode/src/index.ts

@@ -4,7 +4,7 @@ import { Server } from "./server/server"
 import fs from "fs/promises"
 import path from "path"
 import { Bus } from "./bus"
-import { Session } from "./session/session"
+import { Session } from "./session"
 import cac from "cac"
 import { Share } from "./share/share"
 import { Message } from "./session/message"