feat: auto-set project month field on PR open

[kevwilliams]

Summary

• Adds the set-project-month reusable workflow from datum-cloud/actions
• Automatically adds new PRs to the org project (feat: add unix socket listener #60) and sets the Month field to the current calendar month
• Triggers on opened, ready_for_review, and reopened events
• Requires the GitHub App org secrets (DATUM_PROJECT_AUTOMATION_APP_ID, DATUM_PROJECT_AUTOMATION_PRIVATE_KEY)

Test plan

• Open a test PR and verify it appears in project feat: add unix socket listener #60 with the correct Month set