migrate to OpenTofu with Terraform fallback

Add binary lookup in both terraform.go and destroy.go:
tofu preferred, terraform fallback. Update all docs to
reflect the OpenTofu-first approach.

DETAILS.md

@@ -53,7 +53,7 @@ obm/
 │   │   │   └── hetzner_test.go
 │   │   └── provider_test.go
 │   ├── terraform/
-│   │   └── terraform.go           # Runner: Init, Plan, Apply, Destroy wrappers
+│   │   └── terraform.go           # Runner: Init, Plan, Apply, Destroy (OpenTofu/Terraform)
 │   └── validation/
 │       ├── validation.go          # Check interface, Runner, CheckResult, Status enum
 │       └── validation_test.go
@@ -82,7 +82,7 @@ obm/
 | `obm deploy` | `--config <path>` | Interactive walkthrough (default) or non-interactive from YAML |
 | `obm validate` | `--env-file <path>` | Load `.env`, check required vars, validate API keys |
 | `obm status` | — | Show deployment state (not yet implemented) |
-| `obm destroy` | — | Confirmation prompt → `terraform destroy` → state cleanup |
+| `obm destroy` | — | Confirmation prompt → `tofu destroy` → state cleanup |
 | `obm version` | — | Print version with commit hash and build time |
 | `obm help` | — | Print usage |
 
@@ -111,7 +111,7 @@ The `obm deploy` interactive flow runs 8 steps in sequence:
 7. **Tailscale** — Optional VPN setup. Auth key and tailnet domain.
 8. **Discord** — Optional bot integration. Bot token, server ID, user IDs.
 
-Final step: summary display with cost estimate → confirm → write `.env` → optionally run `terraform init && terraform apply`.
+Final step: summary display with cost estimate → confirm → write `.env` → optionally run `tofu init && tofu apply` (or `terraform` if installed).
 
 ### Framework-specific defaults
 
@@ -355,15 +355,15 @@ DigitalOcean prices:
 
 ---
 
-## Terraform Integration
+## Terraform / OpenTofu Integration
 
-`obm` generates the `.env` file that Terraform expects. The actual Terraform configs live in the separate [openboatmobile-ai](https://github.com/openboatmobile/openboatmobile-ai) repo.
+`obm` generates the `.env` file that OpenTofu (or Terraform) expects. The actual infrastructure configs live in the separate [openboatmobile-ai](https://github.com/openboatmobile/openboatmobile-ai) repo.
 
-The `internal/terraform/terraform.go` wrapper provides:
-- `Runner.Init()` — `terraform init -input=false`
-- `Runner.Plan(destroy bool)` — `terraform plan` (with optional `-destroy` flag)
-- `Runner.Apply()` — `terraform apply -auto-approve`
-- `Runner.Destroy()` — `terraform destroy -auto-approve`
+The `internal/terraform/terraform.go` wrapper provides (auto-detects OpenTofu first, Terraform fallback):
+- `Runner.Init()` — `tofu init -input=false`
+- `Runner.Plan(destroy bool)` — `tofu plan` (with optional `-destroy` flag)
+- `Runner.Apply()` — `tofu apply -auto-approve`
+- `Runner.Destroy()` — `tofu destroy -auto-approve`
 
 All commands run in the `WorkDir` and capture combined output.
 
@@ -391,7 +391,7 @@ User sets `TF_VAR_*` env vars → sourced from `.env` → Terraform reads them
 
 ## Destroy Flow
 
-`obm destroy` reads `terraform.tfstate` to list resources, shows them, asks for confirmation, runs `terraform destroy`, then cleans up state files unless `--keep-state` is set.
+`obm destroy` reads `terraform.tfstate` to list resources, shows them, asks for confirmation, runs `tofu destroy`, then cleans up state files unless `--keep-state` is set.
 
 ---