Add jekyll-last-modified-at to populate sitemap lastmod from git history

Sitemap <lastmod> dates now reflect the actual last git commit that
touched each file, instead of just the date from the filename. CI
checkout changed to fetch-depth: 0 so full git history is available.

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

Author: vorushin

.github/workflows/jekyll.yml

@@ -20,6 +20,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
 
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1

CLAUDE.md

@@ -68,8 +68,9 @@ Include in posts via `{% include diagram-name.html %}`.
 ### SEO & Metadata
 
 - `jekyll-seo-tag` generates `<title>`, Open Graph, and Twitter Card meta tags
-- `jekyll-sitemap` auto-generates `sitemap.xml`
+- `jekyll-sitemap` auto-generates `sitemap.xml`; `jekyll-last-modified-at` populates `<lastmod>` from git history
 - `robots.txt` points to the sitemap
 - Default `og:image` set to `/apple-touch-icon.png` via front matter defaults in `_config.yml`
 - Favicons: `/img/favicon.ico`, `/favicon-32x32.png`, `/favicon-16x16.png`, `/apple-touch-icon.png`
 - `404.html` provides a custom not-found page for GitHub Pages
+- CI checkout uses `fetch-depth: 0` (full git history) so `jekyll-last-modified-at` can read commit dates

Gemfile

@@ -5,4 +5,5 @@ gem "jekyll", "~> 4.3"
 group :jekyll_plugins do
   gem "jekyll-seo-tag"
   gem "jekyll-sitemap"
+  gem "jekyll-last-modified-at"
 end

_config.yml

@@ -25,6 +25,7 @@ lang: en
 plugins:
   - jekyll-seo-tag
   - jekyll-sitemap
+  - jekyll-last-modified-at
 
 # Defaults
 # Posts default to lang: ru for legacy Russian archive; English posts override with lang: en