Add tag system, atom.xml test rule, and markdown generation
- Add @page-tags metadata to 39 blog posts across 10 tags (ai, docs-ci,
meta, notebooks, ocaml, odoc, plugins, teaching, tessera, weeknotes)
- Generate tag pages with post lists via gen_blog_index.exe
- @page-tags plugin now uses resolved odoc references for chip links
instead of hardcoded URLs
- gen_atom.ml: deterministic timestamps, CLI args, UTF-8-safe truncation
- gen_rules.ml: add markdown generation phase (@markdown alias), atom.xml
and tag page diff rules under @runtest
- Include weeknotes-2026-15 in blog indexes
- Skip dotfiles in blog scanner (fixes Emacs lockfile crash)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>