Add archive layer: zip, .sfgz share files, locks, snapshots

- Hand-rolled ZIP writer/reader (node:zlib only) with CRC verification,
  UTF-8 names, store/deflate, path-traversal validation; verified
  interoperable with system unzip
- .sfgz guide archives: export, copy-import (fresh ids, remapped
  substeps), linked-import with explicit write-back save
- Advisory .lock-sfgz sidecar locks with stale detection and force-steal
- Snapshot backups with pruning and undoable restore
- 7 more workflow tests (19 total)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/snapshots.js

@@ -0,0 +1,69 @@
+'use strict';
+
+const fs = require('node:fs');
+const path = require('node:path');
+const { zipDirSync, extractZipSync } = require('./zip');
+const { atomicWriteFileSync } = require('./util');
+
+/**
+ * Snapshot backups: a zip of the guide directory (excluding history/) stored
+ * under <guide>/history/snapshots/. Used for automated backups and manual
+ * backup/restore.
+ */
+
+function snapshotsDir(store, guideId) {
+  return path.join(store.guideDir(guideId), 'history', 'snapshots');
+}
+
+function snapshotName(label) {
+  const stamp = new Date().toISOString().replace(/[:.]/g, '-').replace(/-\d{3}Z$/, 'Z');
+  return label ? `${stamp}-${label.replace(/[^A-Za-z0-9_-]+/g, '_')}.zip` : `${stamp}.zip`;
+}
+
+function createSnapshot(store, guideId, { label = '', keepLast = 0 } = {}) {
+  const guideDir = store.guideDir(guideId);
+  if (!fs.existsSync(path.join(guideDir, 'guide.json'))) throw new Error(`guide not found: ${guideId}`);
+  const buf = zipDirSync(guideDir, {
+    filter: (rel) => rel !== 'history' && !rel.startsWith('history/'),
+  });
+  const dir = snapshotsDir(store, guideId);
+  fs.mkdirSync(dir, { recursive: true });
+  const name = snapshotName(label);
+  atomicWriteFileSync(path.join(dir, name), buf);
+  if (keepLast > 0) pruneSnapshots(store, guideId, keepLast);
+  return name;
+}
+
+function listSnapshots(store, guideId) {
+  const dir = snapshotsDir(store, guideId);
+  if (!fs.existsSync(dir)) return [];
+  return fs.readdirSync(dir).filter((n) => n.endsWith('.zip')).sort().reverse();
+}
+
+function pruneSnapshots(store, guideId, keepLast) {
+  const all = listSnapshots(store, guideId);
+  for (const name of all.slice(keepLast)) {
+    fs.rmSync(path.join(snapshotsDir(store, guideId), name), { force: true });
+  }
+}
+
+/**
+ * Restore a snapshot: replaces the guide's current content (guide.json and
+ * steps/) with the snapshot's, keeping the history/ directory intact.
+ */
+function restoreSnapshot(store, guideId, name) {
+  const file = path.join(snapshotsDir(store, guideId), path.basename(name));
+  if (!fs.existsSync(file)) throw new Error(`snapshot not found: ${name}`);
+  const buf = fs.readFileSync(file);
+  const guideDir = store.guideDir(guideId);
+  // Safety: snapshot the pre-restore state too, so a restore is undoable.
+  createSnapshot(store, guideId, { label: 'pre-restore' });
+  for (const entry of fs.readdirSync(guideDir)) {
+    if (entry === 'history') continue;
+    fs.rmSync(path.join(guideDir, entry), { recursive: true, force: true });
+  }
+  extractZipSync(buf, guideDir);
+  return store.getGuide(guideId);
+}
+
+module.exports = { createSnapshot, listSnapshots, pruneSnapshots, restoreSnapshot, snapshotsDir };