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/concept.md

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+---
+type: <Concept type, e.g. BigQuery Table>
+title: <Human-readable display name>
+description: <One-sentence summary>
+resource: <Canonical URI, omit for abstract concepts>
+tags: [<tag>, <tag>]
+timestamp: <ISO 8601, e.g. 2026-06-16T00:00:00Z>
+---
+
+# Schema
+
+| Column | Type | Description |
+|--------|------|-------------|
+| `col`  | TYPE | What it is. FK to [other](/tables/other.md). |
+
+# Citations
+
+[1] [Source title](https://example.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

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

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+# Update Log
+
+## 2026-06-16
+* **Initialization**: Created the bundle structure.
+* **Creation**: Added the [orders table](/tables/orders.md).