docs/ci: add production gateway contract and fix test discovery

[masarray]

Summary

Adds a stack-aware AGENTS.md for ARServer's WPF/.NET IEC 61850 MMS → runtime cache → Modbus TCP / MQTT architecture and fixes a CI test-discovery defect found while validating the branch.

Engineering contract

• root-cause-first workflow with a three-patch circuit breaker
• one authoritative runtime cache; Modbus/MQTT/UI consume coherent snapshots rather than creating separate truth
• typed Result/Try failure contracts for expected protocol/runtime outcomes
• infrastructure exceptions contained at boundaries rather than inner polling loops
• zero UI blocking for MMS/SCL/network/export work
• bounded queues/backpressure for polling, publishing, UI and diagnostics
• async aggregated diagnostics; log failure cannot block gateway operation
• transactional mapping/config promotion and explicit reconnect/session state
• long-running resource/leak and >10% performance-regression review gates

CI root-cause fix

The first PR CI failure was not an ARServer build failure: ARServer.sln built successfully, but recursive test discovery entered .deps/ARIEC61850/tests and invoked dependency test projects with --no-build, even though those test assemblies were not built by the ARServer solution.

CI now:

• keeps the ARIEC61850 dependency checkout required by the ARServer build;
• excludes .deps, bin, and obj from repository-owned test discovery;
• runs only ARServer-owned test projects when present;
• leaves dependency test validation to the dependency's owning repository.

The corrected CI run #82 is green.