Fix gulp-zip import and add consistent modified time to zip (#138)

* Fix yarn zip failing after gulp-zip 6.x update

gulp-zip 6.x switched to ESM exports, so CommonJS require now returns
an object with a default property instead of the function directly.
Also added explicit modifiedTime option for consistent behavior.

Fixes DES-1353

https://claude.ai/code/session_01YXpwhKnsb98MQHXC5c5HLW

* Declare Node >=22.12.0 engine requirement

gulp-zip 6.x is pure ESM, so require('gulp-zip').default only works
on Node 22.12.0+ where synchronous require of ESM is enabled by
default. Make the requirement explicit to surface a clear error on
older Node versions instead of ERR_REQUIRE_ESM.

https://claude.ai/code/session_01YXpwhKnsb98MQHXC5c5HLW

* Remove unnecessary modifiedTime option from zip call

The modifiedTime: new Date() option was speculative and unneeded. The
gulp-zip 6.x default (preserve file stat mtimes) works correctly and
matches the effective pre-6.x behavior.

https://claude.ai/code/session_01YXpwhKnsb98MQHXC5c5HLW

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: minimaluminium

gulpfile.js

@@ -8,7 +8,7 @@ const {mergeLocales} = require('@tryghost/theme-translations/build');
 // gulp plugins and utils
 const livereload = require('gulp-livereload');
 const postcss = require('gulp-postcss');
-const zip = require('gulp-zip');
+const zip = require('gulp-zip').default;
 const concat = require('gulp-concat');
 const uglify = require('gulp-uglify');
 const beeper = require('beeper');

package.json

@@ -4,7 +4,8 @@
     "demo": "https://source.ghost.io",
     "version": "1.6.0",
     "engines": {
-        "ghost": ">=5.0.0"
+        "ghost": ">=5.0.0",
+        "node": ">=22.12.0"
     },
     "license": "MIT",
     "scripts": {