docs(okf): add distilled spec, frontmatter reference, and templates

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

custom-skills/97-ourdigital-okf/code/assets/index.md

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+# Group Heading
+
+* [Title](relative-or-bundle-relative-path) - short description from the concept's frontmatter
+* [Another concept](/tables/orders.md) - one row per completed order
+
+# Subdirectories
+
+* [datasets/](datasets/) - dataset-level concepts