Add rule copy drift check (#3)

README.md

@@ -69,6 +69,14 @@ Cursor, Windsurf, Cline, Copilot, Aider, Kiro: copy the matching rules file from
 
 Kiro: copy `.kiro/steering/ponytail.md` to `~/.kiro/steering/` (global) or `.kiro/steering/` in your project.
 
+## Development
+
+When changing the compact rule text, keep the agent copies aligned:
+
+```bash
+node scripts/check-rule-copies.js
+```
+
 ## FAQ
 
 **Does it need a config file?**

scripts/check-rule-copies.js

@@ -0,0 +1,40 @@
+#!/usr/bin/env node
+const fs = require('fs');
+const path = require('path');
+
+const root = path.join(__dirname, '..');
+
+function read(relPath) {
+  return fs.readFileSync(path.join(root, relPath), 'utf8').replace(/\r\n/g, '\n').trim();
+}
+
+function stripCursorFrontmatter(text) {
+  return text.replace(/^---\n[\s\S]*?\n---\n*/, '').trim();
+}
+
+const agents = read('AGENTS.md');
+const canonical = agents.replace(/\n\n\(Yes, this file also applies[\s\S]*?\)$/, '').trim();
+
+const copies = [
+  ['.cursor/rules/ponytail.mdc', stripCursorFrontmatter],
+  ['.windsurf/rules/ponytail.md', text => text.trim()],
+  ['.clinerules/ponytail.md', text => text.trim()],
+  ['.github/copilot-instructions.md', text => text.trim()],
+];
+
+let failed = false;
+
+for (const [relPath, normalize] of copies) {
+  const actual = normalize(read(relPath));
+  if (actual !== canonical) {
+    console.error(`${relPath} drifted from AGENTS.md`);
+    failed = true;
+  }
+}
+
+if (failed) {
+  console.error('Update the copied rule text or AGENTS.md so the shared body matches.');
+  process.exit(1);
+}
+
+console.log('Rule copies match AGENTS.md shared body.');