fix(cortex-cli): improve --with-api-key help text with usage example

Fixes bounty issue #1392

The help text for the --with-api-key flag now includes a concrete usage
example showing how to pipe an API key from an environment variable,
making it clearer how to use this flag.

Author: factorydroid

cortex-cli/src/main.rs

@@ -245,7 +245,7 @@ struct LoginCommand {
     #[clap(skip)]
     config_overrides: CliConfigOverrides,
 
-    /// Read the API key from stdin
+    /// Read the API key from stdin (e.g., `printenv OPENAI_API_KEY | cortex login --with-api-key`)
     #[arg(long = "with-api-key")]
     with_api_key: bool,