fix: add --output-dir alias to forge train CLI

Added --output-dir as an alias for --output to match user expectations
and documentation. Fixes unrecognized arguments error.

src/companion/forge/train.py

@@ -211,7 +211,9 @@ def main():
         "--data", type=Path, required=True, help="Path to training data JSONL"
     )
     parser.add_argument(
+        "--output-dir",
         "--output",
+        dest="output",
         type=Path,
         default=Path("~/.companion/training"),
         help="Output directory",