fix: differentiate empty --features error from missing --features

Agents couldn't tell whether they forgot --features or passed a
garbage value. Now: missing flag says "requires --features", while
a present but empty-after-parsing value says "value is empty".

Author: fernandomg

source/__tests__/nonInteractive.test.ts

@@ -94,7 +94,7 @@ describe('nonInteractive — validation', () => {
       runNonInteractive({ name: 'my_app', mode: 'custom', features: ',' }),
     ).rejects.toThrow()
     const output = getLastJsonOutput()
-    expect(output.error).toMatch(/--mode custom requires --features/)
+    expect(output.error).toMatch(/--features value is empty/)
   })
 
   it('rejects unknown feature names', async () => {

source/nonInteractive.ts

@@ -77,7 +77,7 @@ function validate(flags: {
   const features = parseFeatures(flags.features)
 
   if (features.length === 0) {
-    fail('--mode custom requires --features. Use --info to see available features.')
+    fail('--features value is empty. Use --info to see available features.')
   }
 
   const invalidFeatures = features.filter((f) => !featureNames.includes(f))