Add 'specs' collection: rfc-app solution designs for in-app review

Seeds the framework's docs/design/*.md as a document collection on the ohm
project (subfolder specs/, unlisted) so the solution designs can be reviewed
and collaborated on in the RFC app until graduated. Dogfood from session
OHM-0090.0.

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

specs/.collection.yaml

@@ -0,0 +1,4 @@
+type: document
+name: Solution Designs
+visibility: unlisted
+initial_state: super-draft