Agent Skills: capturing-knowledge

Captures durable learnings into a second-brain knowledge base (a folder with a conventions.md or KB-declaring CLAUDE.md) when conditions warrant - a reusable insight, a framework crystallizing, a constraint discovered, a decision trail completed. Triggers on "save this to my notes/knowledge base", "file that under", "worth keeping", or the natural pause at the end of substantive work where a real learning emerged. Only operates when a knowledge base exists; not for process chatter, debugging logs, or tactical one-offs.