feat(cli): add flexible positional day argument

- Add parse_date_arg() to utils.py supporting YYYY-MM-DD, MM-DD, and
  DD-MM formats with either - or / as separator
- Add AmbiguousDateError for two-part dates valid as both MM-DD and DD-MM
- Replace --day flag with a positional optional argument (defaults to today)
- Remove old _parse_date() helper from cli.py

src/timesheets/cli.py

@@ -6,7 +6,7 @@ from datetime import date
 from .output import print_summary, write_csv
 from .parser import aggregate_rows, filter_rows_by_date, parse_document
 from .projects import load_project_map
-from .utils import format_date
+from .utils import AmbiguousDateError, format_date, parse_date_arg
 
 
 def build_parser() -> argparse.ArgumentParser:
@@ -44,8 +44,12 @@ def build_parser() -> argparse.ArgumentParser:
         default=None,
     )
     parser.add_argument(
-        "--date",
-        help="Date to use in the output (DD/MM/YY). Defaults to today.",
+        "day",
+        nargs="?",
+        help=(
+            "Day to extract timesheets for. Accepts YYYY-MM-DD, MM-DD, or DD-MM "
+            "(- or / as separator). Defaults to today."
+        ),
         default=None,
     )
     parser.add_argument(
@@ -76,26 +80,21 @@ def _resolve_token(args: argparse.Namespace) -> str:
     return token
 
 
-def _parse_date(date_str: str | None) -> date:
-    """Parse DD/MM/YY date string, or return today."""
-    if date_str is None:
-        return date.today()
-    try:
-        from datetime import datetime
-
-        return datetime.strptime(date_str, "%d/%m/%y").date()
-    except ValueError:
-        print(
-            f"Error: invalid date format {date_str!r}, expected DD/MM/YY.",
-            file=sys.stderr,
-        )
-        sys.exit(1)
-
-
 def main() -> None:
     args = build_parser().parse_args()
 
-    target_date = _parse_date(args.date)
+    if args.day is None:
+        target_date = date.today()
+    else:
+        try:
+            target_date = parse_date_arg(args.day)
+        except AmbiguousDateError as e:
+            print(f"Error: {e}", file=sys.stderr)
+            sys.exit(1)
+        except ValueError as e:
+            print(f"Error: {e}", file=sys.stderr)
+            sys.exit(1)
+
     date_str = format_date(target_date)
 
     if args.joplin: