Add an Init option for -lockfile=MODE

### Terraform CLI behavior

`terraform init` supports `-lockfile=MODE`. The currently documented mode is `readonly`, which suppresses dependency lock file changes while still verifying checksums. This is useful in automation that treats `.terraform.lock.hcl` as an explicitly managed input.

### terraform-exec gap

`Terraform.Init` does not currently expose this flag as an `InitOption`. Using `TF_CLI_ARGS_init` as a fallback is also unavailable through `Terraform.SetEnv`, because terraform-exec intentionally prohibits `TF_CLI_ARGS_*`.

### Proposed API

Expose the mode as an Init option, with a name that does not conflict with the existing `LockFile(bool)` option for `terraform providers mirror -lock-file`. For example:

```go
tf.Init(ctx, tfexec.LockFileMode("readonly"))
```

Terraform CLI documentation: https://developer.hashicorp.com/terraform/cli/commands/init#plugin-installation

I plan to submit a focused pull request with command-construction and end-to-end coverage.

Start at Terraform.Init and the existing InitOption handling, especially LockFile(bool), to understand command construction and avoid the naming conflict. Add focused command-construction coverage and end-to-end coverage for the documented readonly mode; done means Terraform.Init accepts the new option and passes -lockfile=MODE to the CLI.

Written by the indexing model from the issue text.