Summary

• Implement regex-based error classifier that maps CLI subprocess stderr output to actionable error categories
• Export ErrorCategory type (retryable | fatal | context_overflow | timeout | aborted) and classifyError() pure function
• First-match-wins semantics: retryable → context_overflow → fatal_auth → default fatal
• 50 tests covering all pattern categories, case insensitivity, default behavior, first-match-wins ordering, and type completeness

Closes #22

Test plan

• All retryable patterns classified correctly (rate limit variants, 429, 503, overloaded, network errors)
• All context overflow patterns classified correctly (context length/window/overflow, token limits)
• All fatal auth patterns classified correctly (401, 403, unauthorized, forbidden, invalid key, authentication)
• Unknown/empty strings default to fatal
• Case-insensitive matching verified
• First-match-wins ordering verified
• timeout and aborted in type but never returned by classifier
• pnpm check passes (format + typecheck + lint)
• Full test suite passes (13043 tests)

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