Background jobs should notify completion without dumping full logs

[JohnnyVicious]

Summary

Background OpenCode jobs currently rely on the user to run /opencode:status and /opencode:result. There is no lightweight completion notification that tells the user a background job finished while avoiding large log/output injection into the Claude conversation.

Desired behavior

When a background review/task finishes, the plugin should surface a small notification such as:

OpenCode task task-abc123 completed. Run /opencode:result task-abc123 for output.

The notification should not paste the full result or full log automatically.

Potential implementation paths

• Add a polling/notification helper that watches tracked jobs and emits a concise completion message.
• Store completion summaries in job data but keep full output behind /opencode:result.
• Make sure notifications are session-aware so one Claude session does not announce another session's job unless explicitly shared.

Upstream reference

Derived from openai/codex-plugin-cc#134.