Add Go-backed HCL parser, auto-detect engine, batch .tf parsing

[ephur]

Summary

• Add native Go-backed HCL parsing with auto-detect and Python fallback.
• Batch-parse .tf files to reduce process overhead and improve compatibility/perf.
• Integrate Go build/tests into Makefile and strengthen docs and tests.

Rationale

• python-hcl2 has compatibility gaps for complex HCL2/Terraform constructs (and even some simple ones)
• HashiCorp’s native parser (via Go) offers better correctness.
• Spawning a process per file is slow; batching reduces overhead on large repos.

What Changed

• HCL abstraction:
  • Added tfworker/util/hcl_parser.py:
  • parse_string, parse_file, and new parse_files (batch).
  • Auto-detects engine: uses Go helper if found; otherwise python-hcl2.
  • Forced override: TFWORKER_HCL_ENGINE=go|python. For go without helper, raises error.
• Refactors:
  • tfworker/commands/config.py: use parse_hcl_string (no direct python-hcl2 import).
  • tfworker/util/terraform_helpers.py:
  • Collect .tf files and call parse_files once.
  • Logs and skips per-file errors; safe per-file fallback if batch fails.
• Go helper:
  • tools/hcl2json:
  • go.mod and main.go; uses github.com/tmccombs/hcl2json/convert.
  • Supports --stdin (single) and --multi (batch) modes.
  • Unit tests, including repo fixtures (versions.tf).
• Tests:
  • tests/util/test_hcl_parser.py: python vs go paths, multi-file, error surfaces.
  • tests/util/test_hcl_engine_parity.py: engine parity integration test on fixture; skips if helper missing.
  • Go tests updated with fixture coverage.
• Makefile:
  • New go-build and go-test targets; integrated into test and ci-test.
  • Build from the tool subdir to avoid module root errors; place binary at repo root (./tfworker-hcl2json).
  • If tools/hcl2json/go.sum is missing, print guidance to run (cd tools/hcl2json && go mod tidy) and skip Go steps.
  • clean removes tfworker-hcl2json.
• Docs (README):
  • Engine selection: auto vs forced modes; TFWORKER_HCL_BIN respected.
  • Batch parsing behavior for required provider discovery.
  • Go development guidelines (commit go.mod/go.sum, tidy instructions, vendoring optional).

Backwards Compatibility

• Default behavior remains unchanged (python-hcl2) unless the Go helper is present or the engine is forced.
• Error handling/logging during required provider discovery mirrors previous behavior.

Performance

• Batch parsing sends all .tf files to a single Go process (--multi) vs spawning per file.
• Python fallback continues per-file but only used when Go helper is unavailable or forced off.

How To Enable Go Parser

• Build: make go-build (requires Go 1.20+; run go mod tidy once if go.sum missing).
• Default auto-detect: ensure tfworker-hcl2json is on PATH or set TFWORKER_HCL_BIN.
• Force: TFWORKER_HCL_ENGINE=go (errors if helper missing) or TFWORKER_HCL_ENGINE=python.

Testing

• Python unit tests include go-path stubs and multi-file cases.
• Engine parity integration test runs if helper is available (found on PATH or at ./tfworker-hcl2json).
• Go unit tests validate simple and real Terraform fixtures.