Honor --pretty for validation error JSON

[irl-dan]

Environment

• Margin 0.4.0
• macOS 26.2 (build 25C56)
• Apple silicon
• Package: Margin-0.4.0-macOS-arm64.pkg
• Package SHA-256: cb85c8f4bda5fa2c43fb775214d9ff5241d2143cf88d34088c3ea2dc975119a5

Summary

The comments validate help states that --pretty pretty-prints JSON output.
The command ignores this option when validation fails.
The failure JSON is compact and uses one line.

Reproduction steps

1. Create invalid.md with this content.

# Invalid fixture

<!-- margin:comments:v1
{ this is not valid JSON }
-->

2. Run this command.

margin comments validate invalid.md --pretty

Actual behavior

The command exits with status 65.
It returns compact JSON on one line.

{"error":{"code":"INVALID_COMMENT_ENVELOPE","message":"The Margin comment envelope is invalid: The data couldn’t be read because it isn’t in the correct format."},"ok":false,"schema":"urn:margin:cli:v1"}

Expected behavior

The command must apply --pretty to success and error JSON.
The error code and exit status must remain unchanged.

Acceptance criteria

• margin comments validate invalid.md --pretty emits indented JSON.
• The output ends with one line feed.
• The command keeps exit status 65.
• The command keeps error code INVALID_COMMENT_ENVELOPE.
• Automated tests cover pretty-printed error output.