fix(runtime): surface typed harness provider failures

[kjgbot]

Recognized model-provider failures currently reach agent handlers as ordinary nonzero harness results, so a handler can replace an actionable quota or authentication error with “exit 1.” The shared runtime now rejects these failures with HarnessProviderError before returning output to the handler. No agent repository changes are needed.

The runtime classifies usage/credit limits, rate limits, authentication failures, context limits, provider request timeouts, and availability errors. Messages use safe templates, with a validated provider-reported reset time when available. Successful runs, OS kills, and unrecognized process failures retain their existing result contract. No automatic retry, credential switching, or paid fallback is introduced.

harness.provider_error and runner.handler.error carry the structured providerFailure; the runner's error text is actionable for existing run-record consumers. Original diagnostics remain accessible on the error as a non-enumerable result. Cloud customer-notice rendering can consume the structured message without duplicating CLI parsing.

Validation: the unchanged-handler regression failed before the fix; all 174 runtime tests and runtime typecheck now pass. Coverage includes Claude/Codex error envelopes, stdout/stderr diagnostics, invalid reset hints, safe serialization, successful/unknown/OS-kill results, no retry, and no task output reaching a success path. No live customer task was rerun.

[coderabbitai]

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[cubic-dev-ai]

All reported issues were addressed across 9 files

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