Add mobile-first design contract and redesign brief

Scaffold design/ for the frontend's first design contract, extracted
as-built from index.html (document-as-is):
- DESIGN.md: 9-section brand brief (dark venture-CRM look, IBM Plex,
  single #3b82c4 accent) + tokens.tokens.json (DTCG, from :root + an
  inline-style census).
- BRIEF.md: the mobile-first redesign packet. Mobile = 4 surfaces
  (Grid, Pipeline, Reminders, Contacts) in a bottom tab bar; the rest
  desktop-only. Grid view-switching first-class; narrow on-the-go edit
  set (name, contacts, notes/comms/outreach log, stage, reminders) +
  create-investor, all via the canonical grid path (Contacts stays
  read-only). Includes a backend-reality callout: no field-level write
  (whole-grid versioned PUT vs the targeted log-communication path),
  stage is a separate two-call opportunities flow, pill removal has no
  undo, dedup typeahead is client-side.
- brand/ assets, inspiration/ provenance.

Wire the AGENTS.md Design line so any agent reads the contract before
UI work.

design/brand/ten31-logo-white.svg

@@ -0,0 +1,43 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg id="Layer_1" data-name="Layer 1" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 722.69 280.85">
+  <defs>
+    <style>
+      .cls-1 {
+        font-family: LTCGoudyOldstylePro-Bold, 'LTC Goudy Oldstyle Pro';
+        font-size: 192px;
+        font-weight: 700;
+      }
+
+      .cls-1, .cls-2, .cls-3 {
+        fill: #fff;
+      }
+
+      .cls-2, .cls-4 {
+        stroke-width: 3px;
+      }
+
+      .cls-2, .cls-4, .cls-3 {
+        stroke: #fff;
+        stroke-miterlimit: 10;
+      }
+
+      .cls-4 {
+        fill: none;
+      }
+
+      .cls-5 {
+        letter-spacing: -.06em;
+      }
+    </style>
+  </defs>
+  <text class="cls-1" transform="translate(120.54 208.45)"><tspan class="cls-5" x="0" y="0">T</tspan><tspan x="120.96" y="0">en31</tspan></text>
+  <g>
+    <polygon class="cls-3" points="95.52 140.42 54.54 154.4 54.54 126.45 95.52 140.42"/>
+    <line class="cls-2" x1="0" y1="140.42" x2="60.54" y2="140.42"/>
+  </g>
+  <rect class="cls-4" x="97.1" y="1.5" width="527.95" height="277.85"/>
+  <g>
+    <polygon class="cls-3" points="721.15 140.42 680.16 154.4 680.16 126.45 721.15 140.42"/>
+    <line class="cls-2" x1="625.62" y1="140.42" x2="686.16" y2="140.42"/>
+  </g>
+</svg>