feat: a skill creating skill as default

assets/dev-brain/purpose.md

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+# Dev Brain purpose
+
+Dev Brain compiles durable, source-backed knowledge from the registered projects.
+
+## Priorities
+
+- Architecture, behavior, decisions, invariants, workflows, and relationships.
+- Small topic pages that answer recurring development questions.
+- Visible uncertainty and exact project provenance.
+
+## Recurring questions
+
+- Where does a behavior live, and what must remain invariant when it changes?
+- Which decisions constrain the current implementation?
+- Which project sources must be rechecked before relying on this page?