feat: add cover-image task to export cover PDF as 300 DPI PNG

Runs Ghostscript on output/cover.pdf to produce output/cover-300dpi.png
for visual QC and sharing. Also wired into the `pnpm all` pipeline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/cover-image.js

@@ -0,0 +1,31 @@
+import { access, stat } from 'fs/promises';
+import { resolve, dirname } from 'path';
+import { fileURLToPath } from 'url';
+import { execFile } from 'child_process';
+import { promisify } from 'util';
+
+const execFileAsync = promisify(execFile);
+const __dir = dirname(fileURLToPath(import.meta.url));
+const root = resolve(__dir, '..');
+const pdfPath = resolve(root, 'output', 'cover.pdf');
+const outPath = resolve(root, 'output', 'cover-300dpi.png');
+
+try {
+  await access(pdfPath);
+} catch {
+  console.error('output/cover.pdf not found — run `pnpm cover` first');
+  process.exit(1);
+}
+
+console.log('Rendering cover to PNG at 300 DPI…');
+
+await execFileAsync('gs', [
+  '-dBATCH', '-dNOPAUSE', '-dQUIET',
+  '-sDEVICE=png16m',
+  '-r300',
+  `-sOutputFile=${outPath}`,
+  pdfPath,
+]);
+
+const { size } = await stat(outPath);
+console.log(`Cover image: output/cover-300dpi.png  (${(size / 1_048_576).toFixed(1)} MB)`);