feat(instructions): enhance Copilot agent and skill authoring guidelines

[diegoitaliait]

Description

Add internal Copilot agent and skill authoring instructions, improve consistency checks, and update related tests. This update aims to clarify boundaries and coherence between agents and skills while minimizing duplication.

Change Type

• Agent update
• Instruction update
• Skill update
• AI policy update (AGENTS.md, copilot-instructions.md)
• Workflow/automation update
• Verification/automation update
• Documentation/configuration only
• Scripts/Utils update
• Other

List of changes

• Introduced new Copilot agent and skill authoring instructions.
• Updated existing instructions to remove deprecated files and enhance clarity.
• Renamed local GitHub instructions overrides for consistency.

Consumer Impact

• Affected customization assets: Copilot instructions override files.
• Backward compatibility notes: None.
• Rollout or migration notes (if any): Ensure to update any references to the renamed instructions.

Testing Instructions

1. Review the new and updated authoring instructions for clarity and completeness.
2. Validate that all related tests pass successfully.

Validation Evidence

• Repository-level checks result: All checks passed.
• Stack-specific checks/tests result: Tests for new instructions executed successfully.
• Security/supply-chain checks result: No issues found.

Breaking Changes

• This PR contains breaking changes

Checklist

• No secrets/tokens/credentials committed
• Repository artifacts remain in English
• Available repository checks executed as applicable
• Workflow action pins use full SHA with adjacent release/tag reference
• References remain repository-agnostic and reusable