Existing frontend: detect, report, then keep or restyle (ticket 03)

- templates/detect_frontend.py: finds plain HTML, React/Vue/Angular/Svelte apps
  and PyQt web views; prints location, stack, main colours, font and every
  token block (CSS custom properties, <style> tags, Tailwind config).
- templates/frontend.md: shared report / keep / restyle procedure, written
  so rpa-dashboard-ai can point to it.
- SKILL.md steps 1, 2 and 6 point to it.
- tests: detection self-check; fixture c-web-existing-frontend; eval entries
  c-keep and c-restyle (new pages only, per-question grading, checks on what
  changed in the existing files), both passing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/fixtures/c-web-existing-frontend/web/index.html

@@ -0,0 +1,26 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+<meta charset="UTF-8">
+<meta name="viewport" content="width=device-width,initial-scale=1">
+<title>Field Ops Portal</title>
+<link rel="stylesheet" href="css/theme.css">
+<link rel="stylesheet" href="css/app.css">
+</head>
+<body>
+<header class="topbar">
+  <h1>Field Ops Portal</h1>
+  <nav><a href="index.html">Jobs</a><button class="button secondary" id="theme-btn">Dark mode</button></nav>
+</header>
+<main class="page">
+  <section class="card">
+    <h2>Today's jobs</h2>
+    <table>
+      <thead><tr><th>Job</th><th>Site</th><th>Status</th></tr></thead>
+      <tbody id="jobs"></tbody>
+    </table>
+  </section>
+</main>
+<script src="js/app.js"></script>
+</body>
+</html>