bug(task): preserve actionable provider errors on non-zero exits #873

Problem

A standalone zeroshot task run can finish with status: failed and error: null even when the provider printed an actionable failure reason.

On current main:

• consumeStderr() writes stderr to the log but does not retain a diagnostic.
• complete() populates error only for the special fatal detector, session-identity failure, or signal termination.
• detectProviderFatalError() is Claude-only and currently recognizes one fatal text pattern.
• A provider-agnostic classifyProviderError() already exists, but the watcher terminal path does not use it.

This was observed with Gemini exiting 1 after printing IneligibleTierError / UNSUPPORTED_CLIENT: the useful cause remained buried in the log while task status exposed no error. Similar ordinary non-zero Claude exits can have the same result.

The provider rejecting a request is external. Zeroshot discarding the reason is the core bug.

Expected behavior

Every unsuccessful provider exit should persist a bounded, sanitized, actionable failure summary and surface it through task status / inspect.

Acceptance criteria

• A non-zero exit without a signal produces a non-empty task error when the provider emitted a diagnostic.
• Terminal handling uses the registered provider adapter's error classifier for every provider.
• The persisted result distinguishes at least retryable vs permanent/unsupported failures.
• Diagnostic capture is bounded and redacts likely credentials/secrets; complete raw output remains in the task log.
• Signal-only failures and existing session-identity failures retain their more specific reasons.
• Successful exits are unchanged.
• Fake-provider tests cover non-zero stderr/stdout failures for at least Claude, Codex, and Gemini, including the no-signal case.
• zeroshot task status and zeroshot inspect show the persisted reason without requiring users to mine the log.

Related

#152 covers cluster-level provider retry/recovery. This issue is the narrower standalone task watcher/status boundary.