Populate comment_str on digest entries for annotations and key takeaways

[allenporter]

Background

In Supernote's UI and data model, content represents the source text / excerpt, while comment_str represents user notes or commentary on that excerpt. The Partner App renders comment_str as a distinct visual note card.

Current Behavior

All AI-generated output is combined into markdown inside content.

Proposed Improvement

• When generating AI summaries or structured reflections, separate the primary excerpt/context (content) from key takeaways, reflections, or action items (comment_str).
• This leverages the Partner App's native card UI for comments/annotations.