Explains features, terms, and concepts from a codebase. Use whenever the user asks to explain, describe, or understand something ("explain X", "what is X", "how does X work"). Also use when starting an implementation, refactor, or test task — invoke as `depth:pamphlet <concept>` before writing code, `depth:novella <concept>` before designing an approach, or `depth:novel <concept>` before a deep refactor. Agent-triggered mode (depth: prefix) skips the interview and returns a structured WHAT/WHERE/HOW/CONNECTS TO block instead of a narrative.