docs: introduce docs/archive, remove finished sprint boards and generated report

- docs/archive/2026/ with index README: DR tabletop drill, workstation
  audits, HA/Ecowitt draft, pre-Borg backup audit, finished windows
  reinstall project docs
- delete weekend sprint boards (content preserved in MASTER_TODO done log
  and git history)
- untrack generated ops/policy-checks/last-report.md and gitignore it
- fix references (CLAUDE.md, docs/README.md, ops/windows-reinstall/README.md)

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

.gitignore

@@ -22,6 +22,9 @@
 **/*.tgz
 **/*.zip
 
+# Generated reports
+ops/policy-checks/last-report.md
+
 # Local/editor noise
 .DS_Store
 Thumbs.db