Build a ranked, citation-backed focus map of a codebase — the files and symbols an architecture actually hangs on — using CodeGraph when available and a dependency-free builtin (regex AST extraction + `rg` cross-file references) otherwise. Use this any time you (the coding agent) need to orient in an unfamiliar or large codebase before making a change: the user says "help me understand this codebase", "what's load-bearing here", "what would break if I change X", "who calls this", "map out this project's architecture", or you're about to touch code you haven't read yet and want to know what matters before scanning everything by hand. This is the same extractor `wiki-update` Step 3b uses internally — this skill exposes it directly so any agent session can call it on demand, not just during a wiki sync.