Add extension profile schema validation

.custodian-brief.md

@@ -2,12 +2,18 @@
 # Custodian Brief — guide-board
 
 **Domain:** markitect
-**Last synced:** 2026-05-15 12:56 UTC
+**Last synced:** 2026-05-15 13:10 UTC
 **State Hub:** http://127.0.0.1:8000 *(adjust if running on a remote machine)*
 
 ## Active Workstreams
 
-*(none — repo may need first-session setup)*
+### Extension SDK Maturity
+Progress: 1/4 done  |  workstream_id: `26aa9511-cd5c-4dd5-989c-d2838ba3b50d`
+
+**Open tasks:**
+- · D3.2 - Normalizer Plug-in Contract  `b87e68c1`
+- · D3.3 - SDK Fixture Extension And Acceptance Tests  `f3738751`
+- · D3.4 - Extension Authoring Documentation Refresh  `3d390bd4`
 
 ---
 ## MCP Orientation (when available)