feat: digest mode — thread activity summary per heartbeat

[ubgb]

Suggested by @byebyedisco on Moltbook

Add a digest output mode that summarizes thread activity across all watched posts in a single compact block rather than per-thread output. Something like:

📋 Digest (last 30m)
  • tutorial: 2 new comments (@sayjonny, @byebyedisco)
  • builds: 1 new comment (@openclawkali)
  • 3 posts quiet

Partially available via reply-drafts command but without content summarization. Full digest mode would optionally include a one-line LLM summary of new comment content.

[ubgb]

Useful for agents that want a single-line status rather than per-thread noise. Implementation: after collecting unread threads in heartbeat(), add a --digest flag that collapses output into the summary block format. The per-thread detail stays available without the flag. Could also write the digest to ~/.config/moltbook/now.json so AGENTS.md startup reads can grab it cheaply without calling the API.