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/lib/registry-io.mjs

@@ -0,0 +1,75 @@
+// registry-io.mjs — Atomic read/write for url-registry.json and report files.
+// Zero dependencies. Uses rename() for atomic writes.
+
+import { readFileSync, writeFileSync, renameSync, existsSync, mkdirSync } from 'node:fs';
+import { join, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const DEFAULT_DATA_DIR = join(__dirname, '..', 'data');
+
+/**
+ * Load the URL registry from disk.
+ * @param {string} [dataDir] — defaults to ../data/ relative to lib/
+ * @returns {object} parsed registry or empty scaffold
+ */
+export function loadRegistry(dataDir = DEFAULT_DATA_DIR) {
+  const path = join(dataDir, 'url-registry.json');
+  if (!existsSync(path)) {
+    return {
+      version: 1,
+      created_at: null,
+      last_poll: null,
+      sitemap_state: {},
+      urls: {},
+    };
+  }
+  return JSON.parse(readFileSync(path, 'utf8'));
+}
+
+/**
+ * Save the URL registry atomically (write to .tmp, then rename).
+ * @param {object} registry
+ * @param {string} [dataDir]
+ */
+export function saveRegistry(registry, dataDir = DEFAULT_DATA_DIR) {
+  ensureDir(dataDir);
+  const path = join(dataDir, 'url-registry.json');
+  const tmp = path + '.tmp';
+  writeFileSync(tmp, JSON.stringify(registry, null, 2) + '\n', 'utf8');
+  renameSync(tmp, path);
+}
+
+/**
+ * Load a JSON report file (change-report.json or discovery-report.json).
+ * @param {string} name — filename without path (e.g. 'change-report.json')
+ * @param {string} [dataDir]
+ * @returns {object|null} parsed JSON or null if not found
+ */
+export function loadReport(name, dataDir = DEFAULT_DATA_DIR) {
+  const path = join(dataDir, name);
+  if (!existsSync(path)) return null;
+  try {
+    return JSON.parse(readFileSync(path, 'utf8'));
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Save a JSON report file atomically.
+ * @param {string} name
+ * @param {object} data
+ * @param {string} [dataDir]
+ */
+export function saveReport(name, data, dataDir = DEFAULT_DATA_DIR) {
+  ensureDir(dataDir);
+  const path = join(dataDir, name);
+  const tmp = path + '.tmp';
+  writeFileSync(tmp, JSON.stringify(data, null, 2) + '\n', 'utf8');
+  renameSync(tmp, path);
+}
+
+function ensureDir(dir) {
+  if (!existsSync(dir)) mkdirSync(dir, { recursive: true });
+}