Skip PR review workflow for Dependabot PRs

Dependabot PRs don't have access to the secrets required by the
cagent-action reusable workflow (CAGENT_ORG_MEMBERSHIP_TOKEN), causing
the org membership check to fail with "github-token not supplied".

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Guillaume Lours <glours@users.noreply.github.com>