Add Terragrunt support (single + multi-module)

[patrickchugh]

Summary

• Auto-detects Terragrunt sources via terragrunt.hcl presence — no new CLI flags
• Single-module: delegates to terragrunt init/plan/show/graph, feeds into existing pipeline
• Multi-module: discovers child modules, runs per-module plans, merges with module.<name>. prefixes, parses dependency blocks with python-hcl2 to inject cross-module edges into graphdict
• Cross-module links discovered automatically by existing add_relations() code
• Graceful degradation: HCL parse failures skip cross-module links with a warning, diagram still renders

Resolves #114

Files Changed

• modules/tgwrapper.py — new Terragrunt wrapper (detection, plan execution, merging, dependency parsing)
• modules/validators.py — added is_terragrunt_source() API
• modules/interpreter.py — guard for unknown module names in variable_map
• terravision/terravision.py — Terragrunt detection branch in compile_tfdata()
• .github/workflows/lint-and-test.yml — install Terragrunt on CI runners
• Test fixtures + 33 new tests (25 unit + 8 detection)

Test plan

• 340 tests pass locally (including 1 slow Terragrunt integration test)
• Zero regression in existing Terraform-only test suite
• CI runners install Terragrunt and run all tests

🤖 Generated with Claude Code