Add AGENTS.md with Cursor Cloud development instructions

[danipen]

Adds AGENTS.md with Cursor Cloud-specific development instructions for future agents working on this codebase.

What's included

• Prerequisites (.NET 8.0 SDK requirement)
• Key commands table (restore, build, test, demo, benchmark)
• Non-obvious notes (CI uses .NET 6 but projects target net8.0, Onigwrap native version pinning, no linter configured)

Environment verification

All setup steps were verified:

• Build: dotnet build — 0 warnings, 0 errors across all 6 projects
• Tests: dotnet test — 726/726 passed
• Demo: dotnet run on TextMateSharp.Demo — successfully syntax-highlighted sample C# code

Test output:
test_output.log

Demo output:
demo_output.log

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