Add external extension acceptance path

.custodian-brief.md

@@ -2,16 +2,12 @@
 # Custodian Brief — guide-board
 
 **Domain:** markitect
-**Last synced:** 2026-05-15 12:43 UTC
+**Last synced:** 2026-05-15 12:56 UTC
 **State Hub:** http://127.0.0.1:8000 *(adjust if running on a remote machine)*
 
 ## Active Workstreams
 
-### Assessment Operations Baseline
-Progress: 5/6 done  |  workstream_id: `fc5b1573-91b2-4a19-b6a9-dd4d17057d9b`
-
-**Open tasks:**
-- · D2.6 - External Extension Acceptance Path  `65fbf1df`
+*(none — repo may need first-session setup)*
 
 ---
 ## MCP Orientation (when available)