fix: preserve model for exit notifications

[TTTPOB]

Summary

• read the parent session's current model before sending a PTY exit notification
• pass its provider, model ID, and variant to promptAsync
• retain the existing notification behavior when an older OpenCode server does not expose Session.model

Problem

sendExitNotification() currently passes the originating agent but omits the model and variant. OpenCode resolves a prompt without an explicit model as:

input.model ?? agent.model ?? currentModel(sessionID)

If the agent's configured default differs from the model selected for the current session, the generated <pty_exited> user turn switches the session to that default. All subsequent assistant turns then use the wrong model.

This uses only the standard OpenCode session.get and promptAsync APIs. It does not depend on another plugin. The local compatibility cast is needed because the package's pinned SDK type predates the runtime Session.model field.

Verification

• bun test test/notification-manager.test.ts
• bun typecheck
• bun lint
• bun format
• fork CI: unit tests, E2E tests, typecheck, lint, format, and Nix checks
• live OpenCode 1.18.3 run: a gpt-5.6-terra/high session received a <pty_exited> notification and retained gpt-5.6-terra/high on the notification user message, subsequent assistant messages, and session summary

[TTTPOB]

need to mention the reason why i need this is:
i wrote a plugin that enable the main agent choose model and variant dynamiclly when creating subagent, so it may be different from default model in agent config.

[shekohex]

LGTM, thanks!