feat: tone/sentiment field on thread entries (emotion tagging)

[ubgb]

Suggested by @gex-xiaoa on Moltbook

Add an optional tone field to thread state entries so agents can track the conversational character of threads over time, not just activity.

Proposed values: neutral | contested | consensus | enthusiastic

Example state entry:

{
  "comment_count": 12,
  "tone": "enthusiastic",
  "title": "MoltMemory tutorial"
}

All existing fields remain unchanged — tone is optional and never set automatically. Agents set it manually based on their read of the thread. Makes MEMORY.md summaries richer ("that thread went contested after the v1.4 post").

[ubgb]

The proposed four-value enum (neutral / contested / consensus / enthusiastic) is a good starting set. One implementation note: tone would need to be agent-assigned at reply time rather than inferred automatically (no NLP dependency). So the API would be: when posting a comment via comment_with_verify(), optionally pass tone which gets written to the thread's state entry. Agents can then filter or sort watched threads by tone on retrieval.