emacsPackages.b4-review-mode: init at 0.15.2

b4's `misc/emacs/b4-review-mode.el` provides a major mode that
highlights the `b4 review` reply editor (autoloading for *.b4-review.eml
files). Package it via melpaBuild, sourced from the b4 package's
`passthru.src-misc` so the version stays in lockstep with b4 itself,
mirroring the b4-review-vim plugin.

Assisted-by: Claude Code (Claude Opus 4.8)

pkgs/applications/editors/emacs/elisp-packages/manual-packages/b4-review-mode/package.nix

@@ -0,0 +1,19 @@
+{
+  lib,
+  b4,
+  melpaBuild,
+}:
+melpaBuild {
+  pname = "b4-review-mode";
+  inherit (b4) version;
+
+  src = b4.src-misc;
+  sourceRoot = "${b4.src-misc.name}/misc/emacs";
+
+  meta = {
+    description = "Emacs major mode with highlighting for the b4 review reply editor";
+    homepage = "https://git.kernel.org/pub/scm/utils/b4/b4.git/about";
+    license = lib.licenses.gpl2Only;
+    maintainers = with lib.maintainers; [ fzakaria ];
+  };
+}