enhancement: add machine-only JSON output mode for non-interactive REPL

[rlippmann]

Summary

Add a future enhancement path for non-interactive REPL usage that emits machine-only JSON output suitable for automation and integration workflows.

Problem

Current REPL output is primarily optimized for human readability. For scripts and pipeline integrations, callers may want a stable machine-only output mode instead of mixed human-oriented formatting.

Proposed behavior

• Add an explicit CLI flag (for example, --json) that forces non-interactive REPL output to machine-only JSON.
• Keep human-readable output as the default behavior.
• Ensure the JSON output format is documented and treated as stable for scripting use.

Scope notes

• This is a future enhancement.
• Do not change default interactive UX in this issue.
• Do not introduce new export/state commands in this issue.

Motivation

This gives us a clear path to support automation safely without degrading the default REPL experience for humans.

Follow-up boundary notes (initial state vs checkpoint)

• Keep this issue focused on output mode (--json) only.
• For CLI/scripting preload semantics, treat these as separate concerns:
  • Initial authoritative state: load via engine state JSON (import_json / current equivalents).
  • Runtime continuation: load via checkpoint (import_checkpoint) when exact in-flight continuation is needed.
• Checkpoint should not replace authoritative state transport for normal scripting pipelines.
• If preload flags are added later, keep them explicit and separate (for example, --initial-state-* vs --initial-checkpoint).

[rlippmann]

Future direction note:

• Keep REPL human-readable by default.
• For programmatic serialization workflows, prefer the engine-level JSON APIs (to_json / from_json, or their current equivalents) as the canonical contract.
• If we add a machine-readable REPL mode later (for example --json), it should align with those engine APIs rather than introducing a separate schema surface.