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/.gitignore

@@ -20,3 +20,4 @@ node_modules/
 # Runtime state
 .work/
 org/
+scripts/kb-update/data/