Add memory export to JSON/CSV format

[singularityjason]

Description

Add a CLI command or tool to export memories to standard formats (JSON, CSV) for analysis or backup.

Acceptance Criteria

• CLI command: lightning-memory export --format json (or csv)
• Outputs all memories with metadata, timestamps, and types
• Optional filters: --type, --since, --limit
• Works independently of Nostr sync (pure local export)

Context

Currently memory_export exports as NIP-78 Nostr events. A plain JSON/CSV export would be useful for users who want to analyze their agent's memory in spreadsheets or data tools.

[P-r-e-m-i-u-m]

Hi @singularityjason, I would like to work on this issue. Could you please assign it to me?