RENAME clip.py to cli.py & RENAME video.py to core.py to better reflect purpose

In addition, added basic mark command

rewind/cli.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python3
+import sys
+import argparse
+from rewind.core import clip, mark
+
+def build_parser() -> argparse.ArgumentParser:
+    parser = argparse.ArgumentParser(
+        prog="rewind",
+        description="Control OBS recording and create instant clips",
+    )
+
+    sub = parser.add_subparsers(dest="command", required=True)
+
+    save = sub.add_parser("save", help="Save a section from the current recording")
+    save.add_argument(
+        "-s", "--seconds",
+        type=int,
+        default=30,
+        help="Number of seconds to include in the clip (default: 30)"
+    )
+
+    mark = sub.add_parser("mark", help="Mark the current time in the recording for later reference")
+    mark.add_argument(
+        "-n", "--name",
+        type=str,
+        required=True,
+        help="Name of the marker"
+    )
+
+    return parser
+
+def main(argv=None) -> int:
+    parser = build_parser()
+    args = parser.parse_args(argv)
+
+    if args.command == "save":
+        clip(args.seconds)
+    elif args.command == "mark":
+        mark(args.name)
+    else:
+        parser.error("Unknown command")
+
+    return 0
+
+if __name__ == "__main__":
+    sys.exit(main())