fix(cli): add common user agent examples to scrape --user-agent help

Fixes bounty issue #1410

The --user-agent flag help text was too minimal ('Custom user agent string')
without providing any guidance on common values. Added examples of Chrome,
Googlebot, and curl user agents to help users understand common options.

Author: factorydroid

cortex-cli/src/scrape_cmd.rs

@@ -56,7 +56,12 @@ pub struct ScrapeCommand {
     pub timeout: u64,
 
     /// Custom user agent string.
-    #[arg(long)]
+    ///
+    /// Common examples:
+    /// - Chrome (default): 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) Chrome/120.0.0.0'
+    /// - Googlebot: 'Googlebot/2.1 (+http://www.google.com/bot.html)'
+    /// - curl: 'curl/8.0.0'
+    #[arg(long, value_name = "USER_AGENT")]
     pub user_agent: Option<String>,
 
     /// Custom headers (format: "Header-Name: value").