feat(ms-ai-architect): sitemap-based KB change detection system

Adds a zero-dependency Node.js pipeline that polls Microsoft Learn sitemaps
weekly to detect when source documentation changes. Replaces the broken
mtime-based staleness check (all files had identical mtime after release).

Components:
- build-registry.mjs: extracts 1342 URLs from 387 reference files
- poll-sitemaps.mjs: streams ~18 child sitemaps, matches against registry
- report-changes.mjs: prioritized change report (critical/high/medium/low)
- discover-new-urls.mjs: finds relevant new MS Learn pages not yet covered
- run-weekly-update.mjs: orchestrator with --force/--discover/--dry-run

Integration:
- session-start hook reads change-report.json instead of broken mtime check
- hook triggers background poll if >7 days since last check
- generate-skills --update reads change report for targeted MCP updates

Current stats: 69% match rate (924/1342 URLs tracked via sitemaps).
~31% unmatched due to Microsoft URL restructuring (ai-foundry/openai paths).

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

plugins/ms-ai-architect/scripts/kb-update/build-registry.mjs

@@ -0,0 +1,87 @@
+#!/usr/bin/env node
+// build-registry.mjs — Build URL registry from existing reference files.
+// Extracts all learn.microsoft.com URLs and maps them to their source reference files.
+// Usage: node build-registry.mjs [--merge]
+//   --merge: preserve existing sitemap_lastmod data, only add new URLs
+
+import { readdirSync, readFileSync, existsSync } from 'node:fs';
+import { join, relative, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { normalizeUrl, extractUrls } from './lib/url-normalize.mjs';
+import { loadRegistry, saveRegistry } from './lib/registry-io.mjs';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const PLUGIN_ROOT = join(__dirname, '..', '..');
+const SKILLS_DIR = join(PLUGIN_ROOT, 'skills');
+const merge = process.argv.includes('--merge');
+
+// Walk directory recursively for .md files
+function walkMd(dir) {
+  const results = [];
+  if (!existsSync(dir)) return results;
+  for (const entry of readdirSync(dir, { withFileTypes: true })) {
+    const full = join(dir, entry.name);
+    if (entry.isDirectory()) {
+      results.push(...walkMd(full));
+    } else if (entry.name.endsWith('.md') && entry.name !== 'SKILL.md') {
+      results.push(full);
+    }
+  }
+  return results;
+}
+
+// --- Main ---
+const existing = merge ? loadRegistry() : null;
+const urlToFiles = new Map(); // normalizedUrl → Set<relativePath>
+let totalFiles = 0;
+
+const skillDirs = readdirSync(SKILLS_DIR, { withFileTypes: true })
+  .filter(d => d.isDirectory())
+  .map(d => d.name);
+
+for (const skill of skillDirs) {
+  const refsDir = join(SKILLS_DIR, skill, 'references');
+  const files = walkMd(refsDir);
+
+  for (const file of files) {
+    totalFiles++;
+    const content = readFileSync(file, 'utf8');
+    const urls = extractUrls(content);
+    const relPath = relative(PLUGIN_ROOT, file);
+
+    for (const url of urls) {
+      if (!urlToFiles.has(url)) urlToFiles.set(url, new Set());
+      urlToFiles.get(url).add(relPath);
+    }
+  }
+}
+
+// Build registry
+const today = new Date().toISOString().split('T')[0];
+const registry = {
+  version: 1,
+  created_at: today,
+  last_poll: merge ? existing?.last_poll || null : null,
+  sitemap_state: merge ? existing?.sitemap_state || {} : {},
+  urls: {},
+};
+
+for (const [url, files] of urlToFiles) {
+  const prev = merge ? existing?.urls?.[url] : null;
+  registry.urls[url] = {
+    sitemap_lastmod: prev?.sitemap_lastmod || null,
+    reference_files: [...files].sort(),
+    status: prev?.status || 'unpolled',
+  };
+}
+
+saveRegistry(registry);
+
+// Stats
+const multiRef = [...urlToFiles.values()].filter(s => s.size > 1).length;
+console.log(`Registry built: ${urlToFiles.size} unique URLs from ${totalFiles} files`);
+console.log(`  URLs referenced by multiple files: ${multiRef}`);
+if (merge && existing?.urls) {
+  const newUrls = [...urlToFiles.keys()].filter(u => !existing.urls[u]).length;
+  console.log(`  New URLs added (merge): ${newUrls}`);
+}