feat(skills): Add system-commandline-cli skill for managing .NET CLI commands based on System.CommandLine library

[massimobonanni]

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• I have read and followed the Guidance for submissions involving paid services.
• My contribution adds a new instruction, prompt, agent, skill, workflow, or canvas extension file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, skill, workflow, or canvas extension with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.
• I am targeting the main branch for this pull request.

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Description

This pull request adds a new skill for working with .NET CLI applications built using System.CommandLine, and documents its usage and conventions. The most important changes are:

New Skill Addition:

• Added the system-commandline-cli skill, which provides detailed rules, patterns, and best practices for adding, modifying, or reviewing CLI commands in .NET projects using System.CommandLine. The skill includes when to trigger, what scenarios are in scope, and when not to use it. [1] [2]

Documentation Updates:

• Updated docs/README.skills.md to include the new system-commandline-cli skill, its installation command, and a clear description of its intended use cases and exclusions.

Skill Guidelines and Best Practices:

• Created skills/system-commandline-cli/SKILL.md with comprehensive guidelines covering project architecture, command base class usage, options/arguments definition, handler patterns, command grouping, registration, destructive operation confirmation, dependency injection, naming conventions, global options, validation, and a checklist for new commands.

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• New agentic workflow.
• New canvas extension.
• Update to existing instruction, prompt, agent, plugin, skill, workflow, or canvas extension.
• Other (please specify):

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[github-actions]

🔒 PR Risk Scan Results

Scanned 1 changed file(s).

Severity	Count
🔴 High	0
🟠 Medium	0
ℹ️ Info	0

✅ No matching risk patterns were detected in changed files.

This is an automated soft-gate report. Findings indicate review targets and do not block merge by themselves.

[github-actions]

🔍 Vally Lint Results

✅ All checks passed

Scope	Checked
Skills	1
Agents	0
Total	1

Severity	Count
❌ Errors	0
⚠️ Warnings	0
ℹ️ Advisories	0

Summary

Level	Finding
ℹ️	✅ system-commandline-cli (2/2 checks passed)
ℹ️	✓ [spec-compliance] All 1 skill(s) are spec-compliant.
ℹ️	✓ spec-compliance: All spec checks passed.
ℹ️	✓ [valid-refs] All file references across 1 skill(s) are valid.
ℹ️	✓ valid-refs: All file references resolve to existing files within the skill directory.
ℹ️	1 skill(s) linted, 1 passed

Full linter output

### Linting skills/system-commandline-cli
✅ system-commandline-cli (2/2 checks passed)
    ✓ [spec-compliance] All 1 skill(s) are spec-compliant.
        ✓ spec-compliance: All spec checks passed.
    ✓ [valid-refs] All file references across 1 skill(s) are valid.
        ✓ valid-refs: All file references resolve to existing files within the skill directory.

1 skill(s) linted, 1 passed

[Copilot]

Pull request overview

Adds a reusable skill for developing .NET CLIs with System.CommandLine.

Changes:

• Adds command architecture, handlers, DI, validation, and naming guidance.
• Adds the skill to generated documentation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
skills/system-commandline-cli/SKILL.md	Defines the new skill and examples.
docs/README.skills.md	Lists installation and usage details.

Suppressed comments (1)

skills/system-commandline-cli/SKILL.md:313

• The checklist reintroduces the unconditional async requirement even if Rule 3 is corrected. Allow a synchronous handler signature when the command performs no asynchronous work.

5. ✅ Handler signature: `async Task<int> CommandHandler(ParseResult, CancellationToken)`