fix: add dependabot to ai-moderator skip-bots

Dependabot uses GitHub App permissions (`permission: none`), so the
role-based `skip-roles` check doesn't catch it. Add `dependabot` to
`skip-bots` to prevent unnecessary moderator runs on Dependabot PRs.

(claude)

Author: JacobPEvans

workflows/ai-moderator.md

@@ -13,7 +13,7 @@ on:
     types: [opened]
     forks: "*"
   skip-roles: [admin, maintainer, write, triage]
-  skip-bots: [github-actions, copilot]
+  skip-bots: [github-actions, copilot, dependabot]
 
 rate-limit:
   max: 5