Add AGENTS.md for Cursor Cloud development environment

[poroburu]

Adds AGENTS.md with Cursor Cloud specific development instructions including:

• Build, lint, and test commands (referencing existing README and CI docs)
• How to run the server locally in mock mode without hardware HSM
• System dependency note (libssl-dev for openssl-sys crate)
• Key gotchas: secret file permissions, cross-compilation config, hardware/broadcast test gating

Environment verification:

dev_environment_verification.log

All checks pass:

• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — no warnings
• cargo test --all-targets — 56 passed, 5 ignored (hardware/broadcast/CI-only), 0 failed
• Server runs in mock mode on 127.0.0.1:9443 with health, keys, metrics, and auth endpoints working

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