OpenCode 0.15 is out and may support the events / notifications Scion needs to be more fully integrated

## Problem / Motivation

OpenCode is incomplete

## Proposed Solution

OpenCode can be fully Scion featured

## Additional Context

https://github.com/anomalyco/opencode/releases/tag/v1.15.0

Two notable entries from the release notes
- Added an Effect-based core event system for more complete event delivery across sessions and integrations.
- Restored missing event types in the JavaScript SDK, including session and message events.

I do wonder if we need/can make OpenCode more Scion friendly with a custom plugin: https://opencode.ai/docs/plugins/#events

I will be looking into this over the weekend, will report findings and progress here. I have a couple of small PRs here to address a couple gaps I worked through.

Start with the OpenCode 1.15 release notes and the linked plugin events documentation, then compare the new core event system and JavaScript SDK session and message events with Scion's integration needs. The issue does not name Scion files or tests; done would require reporting the findings and defining whether a custom plugin or other integration work is needed.

Written by the indexing model from the issue text.