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-dark.svg

@@ -0,0 +1,22 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 360 90" width="360" height="90">
+  <defs>
+    <radialGradient id="aurora" cx="0.5" cy="0.4" r="0.5">
+      <stop offset="0" stop-color="#f9a8d4" stop-opacity="0.9"/>
+      <stop offset="0.5" stop-color="#c084fc" stop-opacity="0.7"/>
+      <stop offset="1" stop-color="#818cf8" stop-opacity="0"/>
+    </radialGradient>
+    <linearGradient id="stone" x1="0" y1="0" x2="1" y2="0">
+      <stop offset="0" stop-color="#94a3b8"/>
+      <stop offset="1" stop-color="#475569"/>
+    </linearGradient>
+  </defs>
+  <g transform="translate(8 8) scale(0.3125)">
+    <ellipse cx="120" cy="50" rx="76" ry="48" 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="#334155"/>
+    <circle cx="70" cy="30" r="1.5" fill="#f9a8d4"/>
+    <circle cx="155" cy="18" r="1.8" fill="#ffffff"/>
+  </g>
+  <text x="100" y="58" font-family="Iowan Old Style, Charter, Georgia, serif" font-size="36" font-weight="500" fill="#f0f6fc" letter-spacing="-0.01em">Obelisk</text>
+</svg>