feat(rulesets): disable linear history and configure gitflow merges

[JacobPEvans-personal]

Summary

Disables the strict linear history requirement across the org, configures branch-specific allowed merge methods for gitflow repositories, and refactors ruleset targeting to use native GitHub custom properties.

Changes

• Removed the required_linear_history = true rule globally.
• Created org_gitflow_base ruleset enforcing required signatures on both main and develop for gitflow repos.
• Restricted gitflow main branch to only allow merge commits via config/rulesets-defaults.yml.
• Applied a pull_request rule to gitflow develop branch allowing merge, squash, and rebase methods.
• Refactored all gitflow rulesets to natively target repos using a newly provisioned gitflow: true custom organization property instead of hardcoded repository names.
• Cleaned up obsolete comments across all variables and terraform modules.

Test Plan

• CI passed successfully.
• Ran tofu validate and tofu fmt to ensure the syntax and logic is sound.