Add macOS / Linux platform support to setup guide

[zhaizibin]

Problem

The current README only documents Windows-style paths and PowerShell examples. Users on macOS and Linux encounter friction when adapting the paths and commands.

Proposed change

• Add a docs/setup-macos.md with shell-based setup steps.
• Update README with a platform selector note.
• Verify scripts/validate_skill.py works on Python 3.10+.

Acceptance criteria

• docs/setup-macos.md created
• README updated with platform notes
• CI passes on ubuntu-latest

Good first issue — documentation only.