doc: generate README.md from README.org (Emacs batch ox-md)

Add build.el + pre-commit hook so README.org stays the source and
README.md is regenerated for GitHub/MELPA.  Refresh README (screenshots,
Crapette, use-package, new customs).  Makefile: readme/hooks/info-emacs
targets; ship README.md.

build.el

@@ -0,0 +1,77 @@
+;;; build.el --- Batch Org -> Markdown export for card-games -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2026 Corwin Brust
+;; SPDX-License-Identifier: GPL-3.0-or-later
+
+;;; Commentary:
+
+;; A very small Emacs-batch exporter.  It renders the project's Org
+;; sources to Markdown siblings (foo.org -> foo.md) using the built-in
+;; `ox-md' backend, so GitHub and MELPA -- which render Markdown more
+;; faithfully than Org -- can display them.
+;;
+;; Run it by hand, from the Makefile, or from the git pre-commit hook:
+;;
+;;   emacs -Q --batch -l build.el
+;;
+;; By default it exports the files named in `build-org-files' (README.org).
+;; Set the CARD_GAMES_ORG environment variable to a space-separated list
+;; to override that, e.g. to export more documents.  It never touches
+;; known-games.org (an internal research list) unless you ask for it.
+
+;;; Code:
+
+(require 'org)
+(require 'ox-md)
+
+;; Keep batch mode from blocking on prompts.
+(setq org-confirm-babel-evaluate nil
+      org-export-show-temporary-export-buffer nil
+      ;; A stray or not-yet-committed image link must never abort the run.
+      org-export-with-broken-links 'mark
+      make-backup-files nil)
+
+(defvar build-org-files '("README.org")
+  "Default list of Org files to export to Markdown.
+Overridden by the CARD_GAMES_ORG environment variable when set.")
+
+(defvar build-inhibit-run nil
+  "When non-nil, loading build.el defines helpers but does not export.
+ERT or an interactive session can bind this to exercise the helpers.")
+
+(defun build--targets ()
+  "Return the list of Org files to export.
+Honours the CARD_GAMES_ORG environment variable; falls back to
+`build-org-files'."
+  (let ((env (getenv "CARD_GAMES_ORG")))
+    (if (and env (not (string-empty-p (string-trim env))))
+        (split-string (string-trim env) "[ \t\n]+" t)
+      build-org-files)))
+
+(defun build--export-one (orgfile)
+  "Export ORGFILE to a Markdown sibling.
+Log the outcome; never signal, so one bad file cannot abort the run."
+  (cond
+   ((not (file-readable-p orgfile))
+    (message "build: SKIP %s (not readable)" orgfile))
+   (t
+    (message "build: exporting %s -> markdown" orgfile)
+    (with-current-buffer (find-file-noselect orgfile)
+      (condition-case err
+          (let ((out (org-md-export-to-markdown)))
+            (message "build:   wrote %s" out))
+        (error
+         (message "build:   ERROR exporting %s: %s"
+                  orgfile (error-message-string err))))))))
+
+(defun build--run ()
+  "Export every file in `build--targets' to Markdown."
+  (dolist (orgfile (build--targets))
+    (build--export-one orgfile))
+  (message "build: done"))
+
+(unless (bound-and-true-p build-inhibit-run)
+  (build--run))
+
+(provide 'build)
+;;; build.el ends here