feat: add og-worker for on-demand daily-post og:image cards

Cloudflare Worker (changelog-og) that renders 1200x630 og:image cards
per daily post at request time, fetching the post's frontmatter from the
repo and drawing via workers-og/satori. Wired into base.vto behind the
og_worker_url data flag (empty = fall back to build-time per-repo cards).

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

Author: kt3k

.gitignore

@@ -3,3 +3,7 @@ _cache/
 .cache/
 .env
 .claude/scheduled_tasks.lock
+
+# og-worker (Cloudflare Worker)
+og-worker/node_modules/
+og-worker/.wrangler/

docs/og-image.md

@@ -7,6 +7,9 @@ Social-card images used in `og:image` / Twitter cards, referenced from
   About, and any page without a `repo`.
 - **Per-repo cards** (`/<owner>/<name>/index.png`) — generated at build time for
   every watched repo; used by all of that repo's pages.
+- **Per-daily-post cards** (`og-worker/`) — rendered on demand by a Cloudflare
+  Worker; used by daily article pages when `og_worker_url` is set in
+  `src/_data.yml`.
 
 ## Per-repo cards (generated at build time)
 
@@ -23,6 +26,36 @@ fetched from Google Fonts; OFL-licensed). Satori quirk to know: a percentage
 `width` against an auto-sized parent collapses to 0 — the accent underline
 instead relies on the default `stretch` cross-axis alignment.
 
+## Per-daily-post cards (rendered at request time)
+
+`og-worker/` is a Cloudflare Worker serving
+`GET /<owner>/<name>/<YYYY-MM-DD>.png` (the path mirrors the article URL). Per
+request it fetches the post's markdown from `raw.githubusercontent.com`, reads
+the frontmatter (title, commits, …), renders the card with satori
+(`workers-og`), and caches it at the edge — immutable for published posts, one
+hour for not-yet-published dates. No build step or data store involved, so
+nothing else needs to change when posts are added.
+
+```sh
+cd og-worker
+npm install
+npm run dev      # local: http://localhost:8787/denoland/deno/2026-06-04.png
+npm run deploy   # needs `npx wrangler login` once
+```
+
+After the first deploy, set `og_worker_url` in `src/_data.yml` to the worker's
+URL (e.g. `https://changelog-og.<subdomain>.workers.dev`) — daily article pages
+then point their `og:image` at the worker; while it's empty they fall back to
+the per-repo cards.
+
+workers-og quirks worth knowing (its HTML parser is HTMLRewriter-based):
+
+- HTML entities are never decoded — `&nbsp;` renders literally. Use real
+  characters (NBSP, `·`) instead, and don't HTML-escape interpolated text; angle
+  brackets are swapped for `‹›` lookalikes instead.
+- A childless styled `<div>` still needs an explicit `display: flex`, or parsing
+  mangles the tree and satori rejects it.
+
 ## Site-wide card (`src/og.png`, regenerated by hand)
 
 ### Source