fix: select native Arm64 binary under Rosetta 2

[BartoszBlizniak]

Summary

• detect when the macOS shell is running as x86-64 under Rosetta 2
• select the native macos-arm64 Cloudsmith CLI binary on Apple silicon
• preserve macos-x86_64 selection on genuine Intel Macs and when the Rosetta
  probe is unavailable
• document the Rosetta behavior and cover native, translated, and unavailable
  probe results

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring
• Other (please describe)

Validation

• sh -n install.sh
• shellcheck --shell=sh --severity=style install.sh
• bats tests/detect_target.bats — 9 tests passed
• bats tests/ — 45 tests passed
• git diff --check

Compatibility

Apple silicon Macs running the shell under Rosetta 2 now resolve to
macos-arm64 instead of macos-x86_64. Native Apple silicon and genuine Intel
Mac behavior is unchanged. An explicit --target override still takes
precedence. The Windows-only PowerShell installer is unaffected.

Checklist

• I have added or updated tests where appropriate.
• I have updated documentation and CHANGELOG.md where appropriate.
• I have not included credentials, tokens, or other sensitive data.
• I have called out any breaking change explicitly.

[Copilot]

Pull request overview

Fixes macOS target detection so that Apple silicon machines running an x86_64 shell under Rosetta 2 download/use the native macos-arm64 Cloudsmith CLI binary, while preserving existing behavior on genuine Intel Macs and when the Rosetta probe is unavailable.

Changes:

• Updated install.sh target detection to probe sysctl.proc_translated when uname -m reports x86_64/amd64 on macOS.
• Added Bats coverage for native, translated (Rosetta), and unavailable Rosetta probe outcomes.
• Documented the Rosetta behavior in README.md and noted it in CHANGELOG.md.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
install.sh	Prefer macos-arm64 when Rosetta translation is detected under an x86_64 macOS shell.
tests/detect_target.bats	Adds a sysctl stub and tests covering Rosetta/native/unavailable probe outcomes.
README.md	Clarifies that macos-arm64 applies to Apple silicon shells running under Rosetta 2.
CHANGELOG.md	Notes native Arm64 selection under Rosetta 2 in the unreleased feature list.

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