Enable Renovate for Terraform providers and lockfiles (linux/amd64)

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Summary

• Add .github/renovate.json to enable Renovate for Terraform
• Turn on lockFileMaintenance to refresh .terraform.lock.hcl files weekly (Sunday 3am)
• Scope updates to base branch phorcys420/ci-refactor-3
• Add rule to resolve terraform-module updates from https://registry.coder.com

Notes

• Validates via existing CI which runs terraform init -lockfile=readonly on each template
• Lockfiles already exist per template; Renovate will maintain them via lock file maintenance and provider bumps
• Renovate supports Terraform and lockfile maintenance; nested directories are handled by file matching patterns (**/*.tf). Lockfile updates are done via lockFileMaintenance

Testing

• No runtime changes; configuration-only. Renovate will open PRs for provider updates and lockfile refresh.