docs: Add future enhancements to notification compression

Add potential improvements:
- LLM-based summarization
- Learning from user feedback
- Custom notification types
- Bulk actions on notifications
- Notification grouping and statistics

PROJECT_PLAN.md

@@ -585,6 +585,9 @@ class IPCClient:
 - Learn notification patterns from user feedback
 - Custom notification type definitions
 - Action-based email filtering (e.g., "archive all Renovate emails")
+- Bulk actions on notification emails (archive all, mark all read)
+- Notification grouping in envelope list
+- Statistics on notification emails (counts by type, frequency)
 
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