[codex] Add path-based monorepo CI workflows

[martinhohoff]

Summary

• add path-based GitHub Actions workflows for Python, Terraform, and dbt
• add contributing guidance, CI badges, and local commands that mirror the workflows
• add a backend-safe Terraform CI plan helper and Python tests for repository validation

Validation

• make python-check
• make terraform-fmt
• make terraform-validate
• make terraform-plan-ci
• local dbt parse with a temporary dbt-snowflake environment and placeholder profile