feat: add scheduled sync daemon and TRUNCATE migration mode

- Add sync.py for LAN file sync with atomic writes and mtime-based change detection
- Add watch.py daemon that periodically syncs files and migrates only changed tables
- Add TRUNCATE mode to migrate.py (--truncate flag) to preserve table structure
- Update config.yaml schema with check_interval_minutes and lan_sources
- Update README with new features documentation

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

sync.py

@@ -0,0 +1,89 @@
+"""
+LAN file sync module.
+Detects file changes, copies from LAN UNC paths to local, and determines which tables need migration.
+"""
+
+import os
+import shutil
+
+
+def sync_file(local_path, lan_path):
+    """Sync a single file from LAN to local.
+
+    Returns:
+        (status, error_msg)
+        status: 'copied' | 'updated' | 'skipped' | 'error'
+    """
+    try:
+        if not os.path.exists(lan_path):
+            return 'error', f'LAN path not accessible: {lan_path}'
+
+        lan_mtime = os.path.getmtime(lan_path)
+        local_existed = os.path.exists(local_path)
+
+        if local_existed:
+            local_mtime = os.path.getmtime(local_path)
+            if local_mtime == lan_mtime:
+                return 'skipped', None
+
+        # Ensure parent directory exists
+        os.makedirs(os.path.dirname(local_path), exist_ok=True)
+
+        # Atomic copy: write to .tmp first, then os.replace
+        tmp_path = local_path + '.tmp'
+        try:
+            shutil.copy2(lan_path, tmp_path)
+            os.replace(tmp_path, local_path)
+        except Exception:
+            # Clean up temp file on failure
+            if os.path.exists(tmp_path):
+                os.remove(tmp_path)
+            raise
+
+        if not local_existed:
+            return 'copied', None
+        else:
+            return 'updated', None
+
+    except Exception as e:
+        return 'error', str(e)
+
+
+def sync_all_files(cfg, excel_dir):
+    """Sync all LAN source files to local.
+
+    Returns:
+        {rel_path: (status, error_msg)}
+    """
+    lan_sources = cfg.get('lan_sources', {})
+    results = {}
+
+    for rel_path, lan_path in lan_sources.items():
+        local_path = os.path.join(excel_dir, rel_path)
+        status, error_msg = sync_file(local_path, lan_path)
+        results[rel_path] = (status, error_msg)
+
+    return results
+
+
+def determine_tables_to_migrate(cfg, sync_results):
+    """Determine which tables need migration based on sync results.
+
+    A table needs migration if ANY of its source files were copied or updated.
+
+    Returns:
+        set of table names that need migration
+    """
+    tables_to_migrate = set()
+
+    for table_cfg in cfg.get('tables', []):
+        table_name = table_cfg['table']
+        for src in table_cfg['sources']:
+            file_rel = src['file']
+            if file_rel in sync_results:
+                status, _ = sync_results[file_rel]
+                if status in ('copied', 'updated'):
+                    tables_to_migrate.add(table_name)
+                    break
+
+    return tables_to_migrate