Refactor Blender addon structure and add explicit --zip CLI flag

Move operators and preferences out of __init__.py into dedicated modules.
Fix cyclic import by using proper AddonPreferences pattern. Replace
implicit .zip extension detection in CLI with explicit -z/--zip flag.

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

blender_asset_tracer/cli/pack.py

@@ -97,6 +97,14 @@ def add_parser(subparsers):
         "path structure under the target directory. Paths in blend files are "
         "rewritten to relative paths within this structure.",
     )
+    parser.add_argument(
+        "-z",
+        "--zip",
+        default=False,
+        action="store_true",
+        help="Force output as a ZIP archive, even when the target path does "
+        "not end in '.zip'. The '.zip' extension is appended automatically.",
+    )
 
 
 def cli_pack(args):
@@ -156,12 +164,15 @@ def create_packer(
 
         packer = create_shamanpacker(bpath, ppath, target)
 
-    elif target.lower().endswith(".zip"):
+    elif args.zip:
         from blender_asset_tracer.pack import zipped
 
         if args.compress:
             raise ValueError("ZIP packer does not support on-the-fly compression")
 
+        if not target.lower().endswith(".zip"):
+            target += ".zip"
+
         packer = zipped.ZipPacker(
             bpath, ppath, target, noop=args.noop, relative_only=args.relative_only,
             keep_hierarchy=args.keep_hierarchy,