feat: add repo-tokens GitHub Action with token count badge

Reusable composite action that counts codebase tokens using tiktoken
and generates a shields.io-style SVG badge. Color reflects context
window usage: green (<30%), yellow-green (30-50%), yellow (50-70%),
red (70%+). Badge includes hardcoded link back to repo-tokens.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

repo-tokens/examples/yellow.svg

@@ -0,0 +1,14 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="90" height="20" role="img" aria-label="120k tokens">
+  <title>120k tokens</title>
+  <linearGradient id="s" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient>
+  <clipPath id="r"><rect width="90" height="20" rx="3"/></clipPath>
+  <g clip-path="url(#r)">
+    <rect width="52" height="20" fill="#555"/>
+    <rect x="52" width="38" height="20" fill="#dfb317"/>
+    <rect width="90" height="20" fill="url(#s)"/>
+    <g fill="#fff" text-anchor="middle" font-family="Verdana,Geneva,DejaVu Sans,sans-serif" font-size="11">
+      <text x="26" y="15" fill="#010101" fill-opacity=".3">tokens</text><text x="26" y="14">tokens</text>
+      <text x="71" y="15" fill="#010101" fill-opacity=".3">120k</text><text x="71" y="14">120k</text>
+    </g>
+  </g>
+</svg>