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>

docs/content-model.md

@@ -61,7 +61,8 @@ body_format: html              # html | markdown  (see below)
 | `title` | yes | Plain text; rendered into `<h1>` and `<title>`. |
 | `subtitle` | no | Blog/event lead paragraph (`.article-subtitle`). News has none. |
 | `category` | no | Shown before the date on blog pages. |
-| `date` | yes | ISO date, authoritative for listing order. Sourced from `tools/seo_inject.py` `ARTICLE_DATES`, else extracted from the page. |
+| `date` | yes | ISO date, authoritative for listing order and for the article's schema date. Since U3 this is the source of truth (`tools/content_index.py` builds the date registry from it; the old `ARTICLE_DATES` table in `seo_inject.py` is now only a fallback). |
+| `date_type` | no | `published` (default, omitted) or `modified`. Selects `datePublished` vs `dateModified` in the Article schema. |
 | `date_line` | no | The exact human-readable date line from the page, preserved for fidelity. U2 may instead regenerate it per-language from `date`. |
 | `hero.*` | no | Lead image + caption. `src` is root-relative (`/images/…`). `caption_html` is verbatim (may contain attribution links). |
 | `body_format` | yes | See below. |