ui: staffing co-pilot UX fixes — new-tab reference nav + discoverable collapsibles

Reconciles month-old working-tree UX work (served per-request, live on the demo since
2026-05-30) into git. Implements UX_FIX_PLAN.md's 5 user complaints: reference pages
(Profiler/Architecture/Spec/Onboard/Alerts/Workspaces) open in new tabs; contract-card
collapsible shows a persistent chevron + expand/collapse label in both states; proof.html
gains a substrate-signals chapter. Not authored this session — captured as-is (working
state verified: pages serve 200).

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

mcp-server/onboard.html

@@ -91,6 +91,7 @@ table.preview tr:hover td{background:#0d1117}
   <nav>
     <a href=".">Dashboard</a>
     <a href="console">Walkthrough</a>
+    <a href="profiler">Profiler</a>
     <a href="proof">Architecture</a>
     <a href="spec">Spec</a>
     <a href="onboard" class="active">Onboard</a>