Add building-nostr to ref, flesh out summary

AGENTS.md

@@ -54,9 +54,20 @@ Always run `just all` to validate changes.
 - The narrative should flow naturally — explain *why* before showing *what*
 - Write prose that a developer new to nostr could follow from start to finish
 
-## Reference Implementations
+## Reference Materials
 
-The `ref/` directory contains nostr library implementations used for research:
+The `ref/` directory contains resources used for research, split into two categories.
+
+### Philosophy
+
+| Directory | Description |
+|-----------|-------------|
+| `ref/building-nostr` | High-level ideas about what nostr is and how to work with it — philosophy, not code |
+
+This resource should inform the narrative voice, design philosophy, and conceptual framing
+of chapters, but is not a source of implementation patterns.
+
+### Implementations
 
 | Directory | Language | Description |
 |-----------|----------|-------------|