feat: manage infra IAM repository

[glitchedmob]

Adds the private IAM repository to the shared infrastructure repository configuration.

• Preserve private visibility and apply supported common repository settings.
• Grant Infra Platform Admins and Super Admins admin access and Infra Maintainers maintain access.
• Allow repositories to opt out of rulesets and auto-merge when those features are unavailable for private repositories on the current GitHub plan.

[github-actions]

Terraform plan

Artifacts: https://github.com/sgfdevs/infra-gh/actions/runs/33640614832

Show summary

CHANGE	RESOURCE

Decrypt artifacts:
openssl enc -d -aes-256-cbc -pbkdf2 -in tfplan.enc -out tfplan -pass pass:$OUTPUT_ENCRYPTION_KEY
openssl enc -d -aes-256-cbc -pbkdf2 -in tfplan.txt.enc -out tfplan.txt -pass pass:$OUTPUT_ENCRYPTION_KEY