Add ADR 0005: Go Edition — Dual-Implementation Strategy

Proposes a dual-implementation strategy where Bash remains the canonical
reference implementation and a Go edition is introduced as a compiled
alternative with identical functionality.

Key decisions:
- Clean-room implementation (no tenv/hc-install code reference)
- Shared state via TFENV_CONFIG_DIR defaulting to ~/.tfenv/
- Multi-call binary with hardlinks for distribution
- Embedded PGP keys with override
- Go acceptance test suite as the parity gate
- Output/UX free to diverge and improve over Bash
- Best-in-class logging, no bashlog legacy
- Terraform only, architecture extensible
- Full parity required before first Go release
- Future capabilities vision across 6 categories
- Go edition as innovation pipeline feeding back to Bash

Author: Zordrak