US-4: Pretty JSON Mode

[thm-design]

US-4: Pretty JSON Mode

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

As a developer debugging Cortex manually,
I want to run cortex "test this" --json=pretty and get readable indented JSON,
so that I can inspect the structure without piping through jq.

Acceptance Criteria

• --json=pretty outputs indented JSON blocks separated by blank lines
• Each block is valid JSON that can be parsed independently
• No Rich formatting or ANSI codes in output
• Stderr may still receive non-JSON diagnostic messages
• All event types supported in pretty mode

Technical Notes

• --json=pretty sets json_mode="pretty" in argparse
• JsonFormatter.format_event() switches on mode: ndjson vs pretty
• Pretty mode uses json.dumps(indent=2) with blank-line separators between events
• Stderr diagnostics are allowed (not suppressed)