fix(cortex-cli): document environment variables in help output

Fixes bounty issue #1684

Added ENVIRONMENT VARIABLES section to the main help output documenting:
- CORTEX_AUTH_TOKEN: Authentication token for API access
- CORTEX_HOME: Override home directory (affects all directories)
- CORTEX_CONFIG_DIR: Override configuration directory path
- CORTEX_DATA_DIR: Override data directory path
- CORTEX_CACHE_DIR: Override cache directory path
- CORTEX_API_URL: Override API endpoint URL

Author: factorydroid

cortex-cli/src/main.rs

@@ -45,7 +45,15 @@ use cortex_common::CliConfigOverrides;
 #[command(about = "Cortex - AI Coding Agent", long_about = None)]
 #[command(
     subcommand_negates_reqs = true,
-    override_usage = "Cortex [OPTIONS] [PROMPT]\n       Cortex [OPTIONS] <COMMAND> [ARGS]"
+    override_usage = "Cortex [OPTIONS] [PROMPT]\n       Cortex [OPTIONS] <COMMAND> [ARGS]",
+    after_help = r#"ENVIRONMENT VARIABLES:
+    CORTEX_AUTH_TOKEN    Authentication token for API access
+    CORTEX_HOME          Override home directory (affects all directories below)
+    CORTEX_CONFIG_DIR    Override configuration directory path
+    CORTEX_DATA_DIR      Override data directory path
+    CORTEX_CACHE_DIR     Override cache directory path
+    CORTEX_API_URL       Override API endpoint URL
+"#
 )]
 struct Cli {
     #[clap(flatten)]