feat: Add @codebot help command for instant discoverability

Implements help command that shows all available bot commands with examples and categories.

Users can now type '@codebot help' to see complete command list without reading docs.

tools/ai-review/config.yml

@@ -59,6 +59,7 @@ interaction:
     respond_to_mentions: true
     mention_prefix: "@codebot" # Change this to customize your bot's name!
     commands:
+        - help
         - explain
         - suggest
         - security