Add memory statistics dashboard to CLI

[singularityjason]

Description

Add a lightning-memory stats CLI command that shows a summary of the local memory database.

Acceptance Criteria

• CLI command: lightning-memory stats
• Shows: total memories, breakdown by type, database size, oldest/newest memory
• Shows: top vendors by transaction count (if transaction memories exist)
• Shows: Nostr sync status (last push/pull, relay count)
• Clean terminal output with aligned columns

Context

Users currently have no way to get a quick overview of their memory database without querying individual tools. A stats command gives instant visibility.

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

Hi @singularityjason! I'd like to work on this issue.
Could you please assign it to me?