US-1: Single-Command JSON Output (NDJSON)

[thm-design]

US-1: Single-Command JSON Output (NDJSON)

Parent Epic: #3 (JSON Formatter & Event Schema)

As a developer integrating Cortex into a CI/CD pipeline or another tool,
I want to run cortex "analyze this file" --json and get structured JSON output,
so that I can parse and route the response programmatically.

Acceptance Criteria

• Running cortex "refactor main.ts" --json produces NDJSON on stdout
• No ANSI escape codes or Rich formatting in output
• Pipeline exits with code 0 on success, non-zero on error
• First event is session_start, last event is say
• Each event is a complete, valid JSON object on one line
• Piped input also works: echo "analyze main.ts" | cortex --json

Technical Notes

• Implements the core --json argparse flag (nargs="?" with const="ndjson")
• JsonFormatter handles the NDJSON envelope
• Rich Console.print() skipped when json_mode is set
• agent.run() accepts json_mode param, returns JSON string