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/index.ts

@@ -398,11 +398,9 @@ ${app.git ? await ListTool.execute({ path: app.path.cwd }, { sessionID: input.se
     let text: Message.TextPart | undefined
     const reader = result.toUIMessageStream().getReader()
     while (true) {
-      const result = await reader.read().catch((e) => {
-        if (e instanceof DOMException && e.name === "AbortError") {
-          return
-        }
-        throw e
+      const result = await reader.read().catch(async (e) => {
+        console.log(e)
+        next.metadata.error = e.name
       })
       if (!result) break
       const { done, value } = result

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