chore: prevent package-lock.json version drift permanently

Makes the drift that PR #70 caught impossible to repeat:

- `version-packages` script now runs `changeset version && npm install
  --package-lock-only && check-lockfile-sync`, so the lockfile is always
  regenerated and verified as part of consuming a changeset
- New `scripts/check-lockfile-sync.mjs` compares package.json#.version against
  package-lock.json's root and packages[""] version fields (npm ci does not
  enforce these, which is why the prior drift slipped through CI)
- CI now runs `npm run check:lockfile` on every push/PR — any future drift
  fails the build before merge
- COM workflow in CLAUDE.md collapsed back to a single release-bump step now
  that lockfile sync is automatic

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/check-lockfile-sync.mjs

@@ -0,0 +1,29 @@
+#!/usr/bin/env node
+// Fails if package-lock.json's version fields don't match package.json.
+// Changesets bumps package.json but NOT the lockfile — this catches that drift
+// (the top-level `version` in lockfiles is metadata, so `npm ci` won't flag it).
+
+import { readFileSync } from 'node:fs';
+import { resolve, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const repoRoot = resolve(dirname(fileURLToPath(import.meta.url)), '..');
+const pkg = JSON.parse(readFileSync(resolve(repoRoot, 'package.json'), 'utf8'));
+const lock = JSON.parse(readFileSync(resolve(repoRoot, 'package-lock.json'), 'utf8'));
+
+const expected = pkg.version;
+const rootVersion = lock.version;
+const selfVersion = lock.packages?.['']?.version;
+
+const mismatches = [];
+if (rootVersion !== expected) mismatches.push(`  package-lock.json#.version = ${rootVersion} (expected ${expected})`);
+if (selfVersion !== expected) mismatches.push(`  package-lock.json#.packages[""].version = ${selfVersion} (expected ${expected})`);
+
+if (mismatches.length > 0) {
+  console.error(`\nLockfile version drift detected (package.json is ${expected}):`);
+  console.error(mismatches.join('\n'));
+  console.error('\nFix: run `npm install --package-lock-only` and commit the updated package-lock.json.\n');
+  process.exit(1);
+}
+
+console.log(`Lockfile in sync with package.json (${expected}).`);