US-5: REPL Guard

[thm-design]

US-5: REPL Guard

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

As a user,
I want to get a clear error if I try to combine --json with REPL mode,
so that I dont get a broken hybrid output.

Acceptance Criteria

• Running cortex --json without a message (entering REPL mode) produces error event with code "REPL_INCOMPATIBLE"
• Error message: "--json is not compatible with interactive REPL mode. Provide a message argument or pipe input."
• Exit code is non-zero
• No interactive session starts

Technical Notes

• Check after args.parse_args(): if args.json is set AND no message AND stdin is a tty → emit error event + sys.exit(1)
• Does NOT enter agent.repl() — exits before agent is instantiated
• Error event is the only output (no session_start)