WIP experiment with 95% vibe coding

test: add comprehensive test coverage and sync documentation master

- Add IPC integration test framework (game/tests/ipc_integration.rs) - Add optional model/tools fields to SessionStarted for backwards compat - Add tests for chat_ui, debug_server/commands, error recovery constants - Document dead code with rationale comments across 6 source files - Sync testing docs in CONTRIBUTING.md, README.md, ipc-protocol.md