Add AGENTS.md and csharp-guidelines agent skill

[dennisdoomen]

Summary

Adds agent-agnostic instructions and a structured agent skill so that any AI coding agent (GitHub Copilot, Cursor, Codex, etc.) can automatically apply these C# guidelines when generating or reviewing code.

Files added

File	Purpose
\AGENTS.md\	Root-level agent instructions — describes the repo structure, rule format, build commands, and points to the skill
.agents/skills/csharp-guidelines/SKILL.md\	Agent skill with YAML frontmatter and a compact rule index (all 114 rules across 9 categories in table form)
.agents/skills/csharp-guidelines/references/*.md\	Per-category detailed reference files (progressive disclosure — loaded only when the agent needs deeper context)

Design decisions

• *\AGENTS.md* follows the agents.md open format, compatible with GitHub Copilot, OpenAI Codex, Cursor, Amp, Jules, and others.
• *\SKILL.md* follows the Agent Skills specification: YAML frontmatter with
  ame\ + \description, compact body under 500 lines, with detailed content split into
  eferences/\ for progressive disclosure.
• The skill \description\ is written to trigger automatically whenever an agent is generating or reviewing C# code.
• Reference files are organised by the same 9 categories already used in the site navigation.