Add memory_list tool and improve CLI stats

[hd719]

Problem

1. No way to browse memories — memory_recall requires a search query. There's no tool to list all memories or browse by category/date. Users can't audit what's stored about them.
2. CLI stats are minimal — openclaw ltm stats just prints the count, same as ltm list. No breakdown by category, age distribution, or storage size.

Proposal

New memory_list tool

memory_list(category?: string, limit?: number, offset?: number)

• List memories optionally filtered by category
• Paginated with offset/limit
• Sorted by recency (newest first)
• Returns id, text, category, importance, createdAt

Enhanced CLI stats

openclaw ltm stats

Should output:

• Total count
• Breakdown by category (preference: 12, fact: 8, etc.)
• Oldest/newest memory dates
• Average importance score
• DB size on disk

Bonus: openclaw ltm export / openclaw ltm import

• Export all memories as JSON for backup/migration
• Import from JSON (with duplicate detection)
• Useful for GDPR data portability