[codex] Use GitHub App token for PRE frontend dispatch

[dmarmon-gesplan]

Summary

• Replaces the PAT-based infra-apps dispatch with actions/create-github-app-token.
• Uses INFRA_APPS_DISPATCH_APP_ID and INFRA_APPS_DISPATCH_PRIVATE_KEY to create a token for gesplan-idi/infra-apps.
• Documents the PRE frontend dispatch setup in README.

Impact

• appVestimenta still only builds and pushes the PRE ECR image.
• No Helm, kubectl, Terraform, PRO, or latest tag usage is introduced.

Validation

• git diff --check