feat(skills): add codex user agent (googleapis#2973)

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🛠️ Fixes #<issue_number_goes_here>

Author: twishabansal

cmd/internal/skills/generator.go

@@ -171,7 +171,9 @@ function prepareEnvironment() {
 		userAgent = "skills-geminicli";
 	} else if (process.env.CLAUDECODE === '1') {
 		userAgent = "skills-claudecode";
-	}
+	} else if (process.env.CODEX_CI === '1') {
+        userAgent = "skills-codex";
+    }
 	mergeEnvVars(env);
 	{{if .OptionalVars}}
 	OPTIONAL_VARS_TO_OMIT_IF_EMPTY.forEach(varName => {

cmd/internal/skills/generator_test.go

@@ -297,6 +297,16 @@ func TestGenerateScriptContent(t *testing.T) {
 				`env[key.substring(prefix.length)] = process.env[key];`,
 			},
 		},
+		{
+			name:       "codex ci script",
+			toolName:   "codex-tool",
+			configArgs: `"--prebuilt", "test"`,
+			mode:       "bin",
+			wantContains: []string{
+				`userAgent = "skills-codex";`,
+				`} else if (process.env.CODEX_CI === '1') {`,
+			},
+		},
 	}
 
 	for _, tt := range tests {