Simplify agents md (#255)

* docs: simplify AGENTS.md for conciseness

Reduce AGENTS.md from ~500 lines to ~90 lines while retaining essential
information for AI coding assistants. Remove verbose JSON examples,
curl commands, detailed tutorials, and redundant explanations that can
be inferred from the codebase.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

* docs: fix nullable setting and remove redundant copilot-instructions

- Correct AGENTS.md to show nullable is enabled (matches Directory.Build.props)
- Remove .github/copilot-instructions.md as AGENTS.md is the single source of truth
Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: pm7y