Feature/opentofu support

[dhararon]

feat(engine): add OpenTofu support via --engine flag

Type of Change

• Bug Fix
• New Feature
• Refactor
• Documentation

Summary

Adds first-class OpenTofu support as a drop-in alternative to
Terraform. Terraform and OpenTofu share an identical CLI for every subcommand TerraVision
relies on (init, workspace, plan, show -json, graph, -v), so the only real
difference is the executable name.

What changed

• New binary resolver in helpers.py (set_tf_binary() / get_tf_binary()):
  auto prefers terraform on PATH and falls back to tofu, or the engine can be forced.
• Replaced the 9 hardcoded "terraform" invocations across tfwrapper.py and
  tgwrapper.py with the resolved binary.
• Generalised check_terraform_version() / check_dependencies() to report the active
  engine (both Terraform and OpenTofu are v1.x, so the existing major-version check holds).
• Exposed --engine [auto|terraform|tofu] on the draw, visualise and graphdata commands.

Why
Many teams have migrated from Terraform to OpenTofu after the licence change. Previously the
binary name was hardcoded, so TerraVision could not run against an OpenTofu-only toolchain.
This is a low-risk change because the two CLIs are command-compatible.

Manual verification
Ran draw end-to-end with --engine tofu (OpenTofu v1.12.1) against a credential-free AWS
fixture — tofu init/plan/show -json/graph all executed correctly and a valid architecture
diagram was rendered.

Terragrunt note: the direct init/show/graph calls in tgwrapper.py now honour
--engine tofu, but Terragrunt invokes its own configured binary internally. To make
Terragrunt drive OpenTofu, also set TG_TF_PATH=tofu (documented in the usage guide).

Checklist

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same
  update/change?
• Have you written Documentation/Tests?
• Have you done your own code-review?
• Have you disclosed any use of AI tools and models with their version?

AI Assistance Declaration

• Tools used: Claude Code
• Model: Claude Opus 4.8 (1M context)
• Scope: wrote the BDD unit tests for the resolver.

Checklist for Changes to Core Features:

• Have you discussed any major revamp with a reviewer/maintainer first? (Minor, additive change —
  defaults to auto, so existing Terraform behaviour is unchanged.)
• Have you ensured your PR is focused on one major improvement and is not trying to do too many changes
  at once?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable, and made sure the new tests PASS? (7
  BDD scenarios in tests/helpers_unit_test.py, 15/15 pass.)
• Have you successfully run all previous system wide tests with your changes locally?
  (470 passed. The 34 failures are pre-existing and environment-specific — my local
  terraform is a tenv wrapper with no version installed, so terraform -v exits 42 and
  the integration/visualise tests that shell out to the CLI fail at preflight. Verified the
  identical failures on upstream main (b06b9c1) without my changes, so they are not a
  regression. The new resolver tests pass 15/15 and black --check modules is clean.)

[patrickchugh]

Very grateful for your PR @dhararon - I also made another set of cosmetic changes after merging your PR where errors messages mention terraform, and now they fetch the binary name and say tofu or terraform as appropriate.