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/session/message.ts

@@ -138,6 +138,7 @@ export namespace Message {
           created: z.number(),
           completed: z.number().optional(),
         }),
+        error: z.string().optional(),
         sessionID: z.string(),
         tool: z.record(z.string(), z.any()),
         assistant: z