refactor(cli): replace positional input argument with --input / -i flag

AGENTS.md

@@ -53,22 +53,22 @@ Do **not** use `pip` or `python` directly.
 
 ```sh
 # Print CSV to stdout (date defaults to today)
-uv run timesheets input.md
+uv run timesheets --input input.md
 
 # Write CSV to a file
-uv run timesheets input.md -o output.csv
+uv run timesheets --input input.md -o output.csv
 
 # Override the date (DD/MM/YY)
-uv run timesheets input.md --date 22/05/26
+uv run timesheets --input input.md --date 22/05/26
 
 # Use a specific project map file
-uv run timesheets input.md --map /path/to/project_map.json
+uv run timesheets --input input.md --map /path/to/project_map.json
 
 # Print a human-readable summary instead of CSV
-uv run timesheets input.md --summary
+uv run timesheets --input input.md --summary
 
 # Read from stdin
-cat input.md | uv run timesheets -
+cat input.md | uv run timesheets --input -
 
 # Fetch today's entries from Joplin (token via env var)
 JOPLIN_TOKEN=your_token uv run timesheets --joplin

src/timesheets/cli.py

@@ -16,9 +16,10 @@ def build_parser() -> argparse.ArgumentParser:
 
     source = parser.add_mutually_exclusive_group(required=True)
     source.add_argument(
-        "input",
-        nargs="?",
+        "--input",
+        "-i",
         help="Path to the markdown file containing the timesheet table, or '-' to read from stdin.",
+        default=None,
     )
     source.add_argument(
         "--joplin",