feat(seo): add robots.txt + sitemap.xml, refresh JSON-LD metadata

GSC showed 0 sitemap submitted and only / indexed. Adds the discoverability
foundation — robots.txt pointing at sitemap.xml, a one-URL sitemap to grow
from — plus softwareVersion / datePublished / dateModified on the
SoftwareApplication JSON-LD so Google sees freshness, and a releases.atom
<link rel="alternate"> so feed-aware crawlers find the release stream.

No marketing-copy changes: v3.1.0 is contributor-facing (design system
locked in DESIGN.md); hero badge and What's-new stay pinned at v3.0.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: savvides

docs/index.html

@@ -11,6 +11,7 @@
   <link rel="icon" type="image/svg+xml" href="favicon.svg">
   <link rel="icon" type="image/png" sizes="512x512" href="favicon.png">
   <link rel="apple-touch-icon" href="favicon.png">
+  <link rel="alternate" type="application/rss+xml" title="idstack releases" href="https://github.com/savvides/idstack/releases.atom">
 
   <!-- Open Graph -->
   <meta property="og:title" content="idstack — Evidence-based instructional design, transparently cited">
@@ -41,6 +42,9 @@
     "url": "https://idstack.org",
     "applicationCategory": "EducationalApplication",
     "operatingSystem": "macOS, Linux, Windows",
+    "softwareVersion": "3.1.0",
+    "datePublished": "2026-04-20",
+    "dateModified": "2026-05-13",
     "offers": {
       "@type": "Offer",
       "price": "0",

docs/robots.txt

@@ -0,0 +1,4 @@
+User-agent: *
+Allow: /
+
+Sitemap: https://idstack.org/sitemap.xml

docs/sitemap.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+  <url>
+    <loc>https://idstack.org/</loc>
+    <lastmod>2026-05-13</lastmod>
+    <changefreq>weekly</changefreq>
+    <priority>1.0</priority>
+  </url>
+</urlset>