chore: add ReviewRouter workflow

[review-router-ai]

This PR installs the ReviewRouter GitHub Actions workflows.

Security defaults:

• uses pull_request, not pull_request_target
• checks out code with persist-credentials: false
• skips secret-backed review for fork pull requests by default
• uses GitHub OIDC for SaaS runtime config
• keeps provider secrets in this repository or organization Actions secrets
• compact mode keeps small caller workflows here and runs versioned ReviewRouter runtime from 777genius/review-router

Workflow files:

• .github/workflows/reviewrouter.yml - pull request review gate
• .github/workflows/reviewrouter-interaction.yml - /rr comment commands and discussion routing