fix(mcp): clean up subprocess listeners on OAuth browser open

[walker83]

Summary

Clean up subprocess.on("error") and subprocess.on("exit") listeners after they fire, and on the success path.

Problem

When the 500ms timer fires (success path), resume(Effect.void) is called but the error and exit listeners on the subprocess are never removed. The closures captured by these listeners prevent GC of the subprocess object and surrounding scope.

Fix

Extract a common cleanup() helper that:

1. Clears the timer
2. Removes both listeners via subprocess.off()
3. Then calls resume()

This ensures all three paths (timer, error, exit) properly clean up.

Changes

• packages/opencode/src/mcp/index.ts: add cleanup helper for subprocess listeners

Checklist

• Code follows project style guidelines
• All exit paths properly clean up