refactor: move environments from vendored module to root module

[ljuboops257]

Summary

• Removes github_repository_environment and github_repository_environment_deployment_policy resources from the vendored child module (modules/terraform-github-repository/) — vendored modules must not be modified
• Re-implements environments at the root module level in main.tf, keyed by "repo:env" to avoid naming collisions across repos
• Updates import blocks to target the new root-level resource addresses
• Adds deployment policy import blocks for generated repos
• Extracted deployment policy for_each logic into named locals (all_env_branch_policies_map, all_env_tag_policies_map) for maintainability

Closes reviewer comment: G-Research#28 (comment)

Test plan

• terraform fmt produces no changes
• terraform validate passes with "The configuration is valid."
• Terraform plan shows no unexpected destroys for environment resources
• All environment resource addresses use root-level github_repository_environment.environment["repo:env"], not module.repository[*].github_repository_environment