fix(rules): stop rendering the skills paragraph's markup as text

`skillsText` is written into the page with `set:html`, because its copy
carries a `<code>.agents/skills/</code>`. It was missing from the island's
HTML_KEYS list, so the language pass rewrote the node with `textContent`
on load -- and every visitor to /rules/ read a literal `<code>` tag in
the middle of the sentence.

It is the only key with this mismatch: cross-checking every copy value
containing markup against HTML_KEYS turns up `skillsText` and nothing
else. Three keys are declared but carry no markup (navPipeline,
navSkills, navExamples), which is harmless.

Also teaches rendered-text-diff.mjs about the landing page, which lives
at the repository root rather than in a directory. It was requesting
/index/index.html and diffing against a 404, which reported a clean four
spans. With the path fixed the landing page really is clean, 36 of 36.

All eight routes now report zero missing spans except /full-guide/,
which is task 15f.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.agents/scripts/rendered-text-diff.mjs

@@ -22,11 +22,15 @@ import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 import { chromium } from 'playwright';
 
+// `index` is the landing page: it lives at the repository root, not in a
+// directory of its own, so it needs a different path on the legacy side.
 const route = process.argv[2];
 if (!route) {
-  console.error('usage: rendered-text-diff.mjs <route>   e.g. full-guide');
+  console.error('usage: rendered-text-diff.mjs <route>   e.g. full-guide, or index');
   process.exit(2);
 }
+const legacyPath = route === 'index' ? 'index.html' : `${route}/index.html`;
+const astroPath = route === 'index' ? '' : `${route}/`;
 
 // The built site expects to be served under the configured base path.
 const staging = mkdtempSync(join(tmpdir(), 'af-rtd-'));
@@ -75,8 +79,8 @@ try {
     return spans;
   };
 
-  const legacy = await grab(`http://localhost:4197/${route}/index.html`);
-  const astro = await grab(`http://localhost:4196/ai-for-dummies/${route}/`);
+  const legacy = await grab(`http://localhost:4197/${legacyPath}`);
+  const astro = await grab(`http://localhost:4196/ai-for-dummies/${astroPath}`);
   await browser.close();
 
   const rendered = new Set(astro);