build(obelisk): let Claude Code search its own session history

- SQLite + FTS5 index over ~/.claude JSONL transcripts.
  - Agent writes JS queries at runtime — same sandbox pattern as workflows.
  - Covers sessions, subagents, workflow executions, tool calls, and full-text search.

.github/assets/obelisk-wordmark-l2.svg

@@ -0,0 +1,23 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 540 96" width="540" height="96">
+  <defs>
+    <linearGradient id="aurora" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#ec4899" stop-opacity="0"/>
+      <stop offset="0.5" stop-color="#a855f7" stop-opacity="0.55"/>
+      <stop offset="1" stop-color="#6366f1" stop-opacity="0.75"/>
+    </linearGradient>
+    <linearGradient id="stone" x1="0" y1="0" x2="1" y2="0">
+      <stop offset="0" stop-color="#475569"/>
+      <stop offset="1" stop-color="#1e293b"/>
+    </linearGradient>
+  </defs>
+  <g transform="translate(92 8) scale(0.333)">
+    <ellipse cx="120" cy="40" rx="56" ry="40" fill="url(#aurora)"/>
+    <polygon points="120,40 106,64 134,64" fill="url(#stone)"/>
+    <polygon points="106,64 120,64 122,212 100,212" fill="url(#stone)"/>
+    <polygon points="120,64 134,64 140,212 122,212" fill="#1e293b"/>
+    <circle cx="80" cy="30" r="1.2" fill="#a855f7"/>
+    <circle cx="150" cy="20" r="1.5" fill="#6366f1"/>
+  </g>
+  <text x="184" y="46" font-family="Iowan Old Style, Charter, Georgia, serif" font-size="38" font-weight="500" fill="#1f2328" letter-spacing="-0.4">Obelisk</text>
+  <text x="184" y="72" font-family="Iowan Old Style, Charter, Georgia, serif" font-size="15" font-style="italic" fill="#6e7681">Let Claude Code search its own memory.</text>
+</svg>