fix: remove incorrect else branch causing flatten command regression

This fixes a regression bug where the flatten command would fail with an
error message even when valid arguments were provided.

The bug was:
- First introduced in commit 0fdbca7
- Fixed in commit fab9d5e (v5.0.0-beta.2)
- Accidentally reintroduced in commit ed53943

The else branch at lines 130-134 was incorrectly handling the case when
users provided arguments, showing a misleading error about
'no arguments provided' when arguments were actually present.

Fixes all flatten command variations:
- npx bmad-method flatten
- npx bmad-method flatten --input /path
- npx bmad-method flatten --output file.xml
- npx bmad-method flatten --input /path --output file.xml

tools/flattener/main.js

@@ -127,11 +127,6 @@ program
           path.join(inputDir, "flattened-codebase.xml"),
         );
       }
-    } else {
-      console.error(
-        "Could not auto-detect a project root and no arguments were provided. Please specify -i/--input and -o/--output.",
-      );
-      process.exit(1);
     }
 
     // Ensure output directory exists