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/contractor.html

@@ -54,7 +54,7 @@ body{font-family:'Inter',-apple-system,system-ui,sans-serif;background:#090c10;c
 </head><body>
 <div class="bar">
   <h1>Staffing Co-Pilot · Contractor Profile</h1>
-  <a href="/">← Dashboard</a>
+  <a href=".">← Dashboard</a>
 </div>
 <div class="content">
   <div class="search-box">