update cagent-action to latest (with better permissions)

[derekmisler]

Summary

Hardens the PR Review workflow by pinning cagent-action to v1.3.1 (from v1.2.13), switching from pull_request_target to pull_request for safer secret handling, and explicitly passing only the secrets the reusable workflow needs instead of secrets: inherit.

Closes: https://github.com/docker/gordon/issues/281

Changes

• Pinned workflow version: Updated docker/cagent-action from @3a12dbd (v1.2.13) to @dba0ca5 (v1.3.1).
• Trigger change: Replaced pull_request_target with pull_request — auto-reviews now run only for same-repo branches (fork PRs use the /review command via issue_comment).
• Scoped permissions: Added inline comments explaining each permission. Added a job-level scoping comment for clarity.
• Explicit secrets: Replaced secrets: inherit with individually named secrets (ANTHROPIC_API_KEY, CAGENT_ORG_MEMBERSHIP_TOKEN, CAGENT_REVIEWER_APP_ID, CAGENT_REVIEWER_APP_PRIVATE_KEY), reducing exposure of unrelated repository secrets.
• Inline documentation: Added comments explaining each trigger, permission, and secret for future maintainers.

Test plan

• Open or re-open a PR against main and verify the review workflow triggers automatically.
• Post a /review comment on a PR and verify the workflow triggers via issue_comment.
• Confirm review comments appear under the configured GitHub App identity (not github-actions[bot]).

[docker-agent]

Assessment: 🔴 CRITICAL

This PR updates the cagent-action workflow with improved security practices, but introduces a critical security vulnerability through the issue_comment trigger.