When a task's steps live only in an unreadable PROMPT.md, updateStep still can't
resolve step 0 — but it now throws an error naming PROMPT.md as the cause instead
of a misleading "task has 0 steps". The operation genuinely cannot succeed (no
step data), so this is accurate reporting, not a silent success. The reachable
reset path (which derives its loop from getTask's steps) remains unaffected.
Adds a test asserting the PROMPT.md-attributed error.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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