Add skip_knowledge flag to filter irrelevant sessions from knowledge base

[notkurt]

Summary

This PR adds a mechanism to flag sessions as not relevant for knowledge ingestion, allowing the AI to explicitly mark test chats, demos, and other non-productive sessions so they don't pollute the project knowledge base.

Key Changes

• AI-driven relevance assessment: Updated the summarization prompt to include a new "Relevance" section where the AI can write "skip" to flag sessions as not worth preserving
• Frontmatter tracking: Sessions flagged as skip are marked with a skip_knowledge: true field in their frontmatter for downstream filtering
• Knowledge base filtering: Modified buildKnowledge() to skip sessions with the skip_knowledge flag and log how many were excluded
• Session listing: Updated the session list display to show "(skipped)" indicator for filtered sessions
• Export utility function: Made addFrontmatterField() public in session.ts to support frontmatter updates in the background processor
• Comprehensive tests: Added test cases covering skip/keep detection, case-insensitivity, and missing Relevance sections

Implementation Details

• The extractSections() function now parses the Relevance section and returns a skipKnowledge boolean flag
• When a session is flagged as skip, the background processor immediately updates the session file's frontmatter before attempting any knowledge ingestion
• All knowledge ingestion logic (decisions, mistakes, auto-detected corrections) is wrapped in a !sections.skipKnowledge guard to prevent processing
• The knowledge builder provides visibility into filtering by logging the count of skipped sessions

https://claude.ai/code/session_01FwyHciZ86DLhgvEH5Gi5Uu