fix(opencode): end sessions on quit via server.instance.disposed

[peyton-alt]

No description provided.

[cursor]

PR Summary

Low Risk
Low risk: adds a shutdown event handler to ensure session-end fires reliably on exit; main risk is potential duplicate/missed session-end if event ordering differs across OpenCode versions.

Overview
Ensures OpenCode sessions are explicitly ended when the process quits by handling the server.instance.disposed event and firing a synchronous session-end hook using the last tracked currentSessionID.

This also clears per-session in-memory state (seenUserMessages, messageStore, currentSessionID) during shutdown to avoid leaving stale tracking data.

^{Written by Cursor Bugbot for commit 5f2edd9. Configure here.}

[Copilot]

Pull request overview

This PR updates the OpenCode agent’s embedded plugin template so Entire reliably emits a session-end hook when OpenCode exits, ensuring sessions are properly closed even when the user quits (rather than explicitly deleting a session).

Changes:

• Handle the server.instance.disposed OpenCode event to end the active session on shutdown.
• Clear per-session in-memory tracking state before emitting session-end.
• Use the synchronous hook invocation path for shutdown-time reliability.