fix(cli): improve --pr-review flag help text to explain automation scope

Fixes bounty issue #1416

The --pr-review flag had minimal documentation that just said 'Include PR review automation'
without explaining what the automation actually does. Updated the help text to clarify that
it posts welcome messages on new PRs, enables the /cortex review command in PR comments,
triggers re-review on new commits, and adds pull_request and pull_request_review event
triggers to the workflow.

Author: factorydroid

cortex-cli/src/github_cmd.rs

@@ -40,7 +40,9 @@ pub struct InstallArgs {
     #[arg(short, long)]
     pub force: bool,
 
-    /// Include PR review automation.
+    /// Enable PR review automation: posts welcome messages on new PRs, enables
+    /// /cortex review command in PR comments, and triggers re-review on new commits.
+    /// Adds pull_request and pull_request_review event triggers to the workflow.
     #[arg(long, default_value_t = true)]
     pub pr_review: bool,