add subprocess-based CLI integration tests for hcl2tojson and jsontohcl2

50 tests across 12 classes verify end-to-end behavior that unit tests
cannot: real exit codes, stdout/stderr separation, stdin piping, pipe
composition (hcl2tojson | jsontohcl2 round-trips against golden fixtures),
NDJSON structured errors, basename collision handling, stdout buffering
with skip, stdin-to-output-file, compact separator correctness, NDJSON
IO vs parse error exit codes, and TTY vs pipe default behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kkozik-amplify