F-19 [Low] No committed tfvars and remote state backend is commented out — `terraform plan` requires prompting; local state risks drift

## Summary\n\n`infra/terraform/environments/` is empty and the remote state backend in `infra/terraform/main.tf` is commented out. Running `terraform plan` prompts for every variable and uses local state, which risks state drift, loss, and conflicts across team members.\n\n## Affected Files\n\n- `infra/terraform/environments/` — empty\n- `infra/terraform/main.tf` — backend block commented out\n\n## Impact\n\n- `terraform plan` cannot run non-interactively in CI.\n- Local state files risk becoming out of sync or being lost.\n- No team member has a documented path to run Terraform correctly.\n\n## Fix\n\n1. Commit example `terraform.tfvars.example` files to `infra/terraform/environments/`.\n2. Configure and document a remote state backend (Azure Storage Account recommended).\n3. Add backend configuration to the Terraform docs/README.\n\n## References\n\n- Audit artifact: `spin-agent-full-audit.md` (project artifact 1e910fcc580a4cb0)\n- Auditor: Thor

Inspect the empty infra/terraform/environments/ directory and infra/terraform/main.tf first, then review the Terraform docs/README. Confirm how variables are currently supplied and how the commented backend is structured. Done means example tfvars files are committed, the remote backend is configured and documented, and Terraform can run non-interactively without relying on local state.

Written by the indexing model from the issue text.