fix: use "not a directory" wording in --path error message

Address review feedback — the error body now says "not a directory"
instead of "is a file", which is more accurate for non-directory
path types.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: byrichardpowell

packages/theme/src/cli/flags.ts

@@ -28,7 +28,7 @@ export const themeFlags = {
       if (!isDirectorySync(resolvedPath)) {
         renderError({
           headline: 'The path must be a directory, not a file.',
-          body: [`The provided path is a file: ${resolvedPath}`, `Did you mean: ${dirname(resolvedPath)}`],
+          body: [`The provided path is not a directory: ${resolvedPath}`, `Did you mean: ${dirname(resolvedPath)}`],
         })
         return process.exit(1)
       }