Migrate rcopy CLI from Bash to Go

[mrAibo]

Summary

• add a dependency-free Go implementation of the rcopy CLI
• preserve the existing Bash implementation as a compatibility fallback
• keep existing command-line options and exit-code behavior
• add unit and Bats regression tests
• add CI checks for formatting, modules, tests, vet, vendor-only build, and static Linux/amd64 build
• publish the rcopy-linux-amd64 workflow artifact

Design

The implementation follows the Ponytail rules: standard library first, no new runtime dependencies, direct command execution without shell interpolation, minimal file layout, and executable regression coverage for non-trivial argument/command construction.

Compatibility note

The Go CLI is introduced alongside rcopy.sh; the Bash function is not removed in this change.