Add structured error catalog with agent recovery guidance

[timfong888]

Problem

OutputContext.fail() returns codes like UPLOAD_FAILED, INIT_METHOD_REQUIRED with a retryable flag — excellent foundation. But there's no error catalog documenting which errors are transient vs permanent.

Details

• upload.ts catches errors generically — agents get UPLOAD_FAILED with a message string but no breakdown (gas issue? provider rejection? PDP failure?)
• Without classification, agents can't make intelligent retry/abort decisions
• The retryable flag exists but isn't consistently documented

Ask

• Document common error codes with descriptions
• Classify each as transient (retry) vs permanent (abort/escalate)
• Add recommended retry strategies (backoff, max attempts)
• This enables agents to handle failures programmatically instead of just surfacing error strings