feat: add repl json output mode

[rlippmann]

What changed

• Added --json CLI flag for machine-readable REPL output in non-interactive usage.
• Implemented NDJSON-style output: one complete JSON object per processed input line.
• Reused existing controller result shapes for JSON step and preview outputs.
• Added JSON envelopes for state command output and command-layer errors.
• Kept default human-readable REPL behavior unchanged.
• Added targeted REPL tests for JSON command paths, error paths, and contract coverage.
• Added a minimal README Quickstart note for --json NDJSON behavior.

Why this was needed

• Provides stable script-friendly REPL output for automation workflows.
• Preserves existing interactive UX while adding machine-readable non-interactive output.
• Establishes a versioned, deterministic output surface for future parity work.

Closes #56