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/share/share.ts

@@ -1,6 +1,6 @@
 import { App } from "../app/app"
 import { Bus } from "../bus"
-import { Session } from "../session/session"
+import { Session } from "../session"
 import { Storage } from "../storage/storage"
 import { Log } from "../util/log"