Add machine-readable CLI JSON output

[mintyPT]

Summary

• Add --json output to render-producing CLI paths including render, templates, presets, config recipe, and config set
• Include path, format, dimensions, warnings, cache status, and duration in JSON summaries
• Keep human diagnostics off stdout when JSON output is requested and emit structured JSON errors for --json failures
• Add focused CLI coverage for render, preset, and config set JSON output

Verification

• npm run check
• Full test matrix left to GitHub CI per repository instructions

Fixes #45