Gaotax2006 [ Fix ] Fix #3: Add --pretty flag for indented JSON output

[Gaotax2006]

Fixes #3

Add --pretty flag that indents JSON output (2-space). Distinct from --human which shows human-readable tables.

Files changed

• src/output.rs
• src/main.rs

Acceptance checklist

• Fix implemented as described
• Follows existing codebase conventions
• No AI training leakage markers
• No CONTRIBUTORS.json modifications
• No build artifacts committed

[Gaotax2006]

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