Add cross-harness general PowerShell AI skills (Copilot, Codex-style agents, Claude)

[Copilot]

This issue asked for reusable, non-repo-specific AI skills for PowerShell development, formatted correctly for common AI harnesses. This PR adds a small, general skill set in the expected folder structures and front-matter formats for Copilot, Codex-style agents, and Claude Code.

• Added general PowerShell skills for Copilot and agent harnesses

  • Added SKILL.md entrypoints under both:
    • .github/skills/*/SKILL.md
    • .agents/skills/*/SKILL.md
  • New skills:
    • powershell-script-cli
    • powershell-module-design
    • pester-test-authoring

• Added matching Claude command skills

  • Added command docs under .claude/commands/*.md:
    • ps-script.md
    • ps-module.md
    • pester.md
  • Each includes YAML front matter (description, argument-hint, allowed-tools) plus concise workflow steps.

• Skill content scope

  • Focused on broadly applicable PowerShell engineering patterns:
    • Script parameter design + pipeline behavior
    • Module API boundaries + exports
    • Pester test structure + deterministic assertions
  • Intentionally avoids repository-specific implementation details.

Example skill front matter format used:

---
name: powershell-module-design
description: Use when creating or evolving reusable PowerShell modules with public/private function boundaries and maintainable exports.
---