refactor: eliminate dead codex_runtime methods, add anti-fallback policy
codex_runtime.dart (-290 lines):
- Remove 17 dead methods behind UnsupportedError guard
(findCodexBinary, startStdio, request, startThread, resumeThread,
sendMessage, interrupt, getAccount, listModels, listSkills, stop,
dispose, _resolveLaunchConfiguration + 3 @visibleForTesting wrappers)
- Remove 10 dead fields (_process, _state, _pendingRequests, _events, etc.)
- Remove ChangeNotifier mixin (nothing to notify)
- Keep only model types, enums, and standalone helper functions
AGENTS.md (+21 lines):
- Add Fallback and Dead Code Elimination Policy section
- Forbidden: cascading fallbacks, lingering DEPRECATED code,
dead code behind guards, silent catch blocks, redundant indirection,
excessive JSON key probing
- Required: inline WHY comments on every retained fallback chain
Additional cleanup:
- gateway_acp_client.dart: remove unused _GatewayAcpSessionUpdate class
- runtime_controllers_entities.dart: replace _canRefreshThroughRuntime
with runtimeInternal.isConnected
- runtime_models_gateway_entities.dart: relocate CollaborationAttachment
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