init: llm wiki scaffold per Karpathy pattern

wiki/index.md

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+# Wiki Index
+
+> Master catalog of all wiki pages. Updated on every ingest.
+
+## Entities
+<!-- pages about people, organizations, tools -->
+
+## Concepts
+<!-- pages about ideas, techniques, theories -->
+
+## Sources
+<!-- pages summarizing original documents, papers, articles -->
+
+## Comparisons
+<!-- pages comparing tools, approaches, or ideas -->
+
+---
+*Last updated: 2026-05-10*