Drop the --prompt/-p flag entirely. Non-interactive mode is now
triggered by passing positional arguments:
kit "Explain this"
kit @file.go "Review this" --json
kit @a.go @b.go --quiet
Updated extension examples (kit-kit.go, subagent-widget.go) to pass
the prompt as a positional arg. Updated AGENTS.md and README.md.
Add a --json flag that outputs structured JSON (response, model, usage,
messages with typed parts) when used with --prompt. Update kit-kit and
subagent-widget extensions to use --json for cleaner subprocess output
parsing instead of raw text heuristics.