feat(cms): U3 derive article dates from content/ front-matter

Replace the hand-maintained ARTICLE_DATES registry as the source of
truth with dates read from content/ front-matter, so publishing a post
via the CMS needs no manual registry edit (plan R4).

- tools/content_index.py: load_dates() reads content/ front-matter into
  {source_folder/slug: {published|modified: iso}}. Standalone (os+yaml),
  no import cycle with seo_inject.
- tools/seo_inject.py: ARTICLE_DATES is now {**static_fallback,
  **content_dates}; content wins. The static table is kept only as a
  fallback for entries not present in content/.
- migration now preserves the published-vs-modified distinction
  (date_type), so the switch is byte-identical: content-derived dates
  equal the old static table exactly (17/17, incl. the one 'modified').

Verified: full `seo_inject --all --sitemap` + build_listings +
build_botlinks run produces ZERO changes to any site page. build_content
--check still reproduces all 51 pages.

build_botlinks.py needs no change (it scrapes built pages, which are a
pure function of content/). Listings get content-driven dates for free
via seo_inject.ARTICLE_DATES.

tools/tests/test_content_index.py: 5 tests (dates == legacy registry,
merged registry unchanged, published/modified distinction, events key on
projects/<slug>, one ISO date per entry).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/README.md

@@ -1,5 +1,19 @@
 # tools/
 
+## content_index.py
+
+Reads the `content/` source tree (front-matter) as structured data. Its
+`load_dates()` builds the article date registry consumed by `seo_inject.py`,
+so **adding a post no longer needs a manual `ARTICLE_DATES` edit** — the date
+lives in the post's front-matter (`date` / `date_type`). See
+`docs/content-model.md`.
+
+## build_content.py
+
+Renders the `content/` source files into the blog/news/event detail pages
+(`--check` verifies the output matches the current site; `--write` emits pages).
+See `docs/content-model.md` and the no-code CMS plan in `docs/plans/`.
+
 ## seo_inject.py
 
 Injects a technical-SEO block into every language page (`en|mk|si/**/index.html`)