Create .clinerules/dotnet-core.md for coding standards

[CornerstoneCode]

Summary

This PR adds a reusable Cline rule that establishes a principal-level protocol for .NET / ASP.NET Core development.

The rule is designed to work with any .NET repository and guides agentic workflows across:

• repository discovery
• planning and scoped implementation
• verification (build/test)
• secure, production-ready delivery

It emphasizes correctness, maintainability, testability, and alignment with existing repository conventions rather than imposing a specific architecture.

Key Characteristics

• Framework-agnostic within the .NET ecosystem (.NET / ASP.NET Core)
• Enforces a structured workflow: Discover → Plan → Implement → Verify → Deliver
• Includes guardrails for async usage, nullability, security, testing, and dependency discipline
• Avoids repo-specific assumptions and works for greenfield and legacy codebases
• Compatible with Cline’s conditional rule loading via path globs

Intended Use

This rule can be:

• added to .clinerules/ in a .NET repository, or
• used as a community prompt for consistent, high-quality agentic behavior across projects.

No functional code changes are included.