US-3: Error Handling in JSON Mode

[thm-design]

US-3: Error Handling in JSON Mode

Parent Epic: #4 (Event Coverage & Error Handling)

As a pipeline consumer,
I want errors to be structured JSON events,
so that I can handle failures programmatically without parsing terminal logs.

Acceptance Criteria

• Missing input produces {"type":"error","data":{"code":"NO_INPUT"}}
• LLM API failures produce {"type":"error","data":{"code":"API_ERROR","message":"..."}}
• Tool execution failures produce {"type":"error","data":{"code":"TOOL_ERROR","message":"...","tool":"read_file"}}
• Error events are the last event emitted (or only event)
• Exit code is non-zero when any error event is emitted
• All errors include ISO timestamp

Technical Notes

• Maps existing print() error handlers to structured JSON events
• Error codes: NO_INPUT, API_ERROR, TOOL_ERROR (future: REPL_INCOMPATIBLE)
• Detail field for additional context (stack trace, status code)