refactor(reporting): split reporting.py into sub-package (518 > 500-line gate)

[Alberto-Codes]

Context

src/docvet/reporting.py is at 518 lines after Story 32.4 added the suppressed parameter to format_json() (+18 lines). This exceeds the 500-line module size gate.

Proposed Split

The module has natural seams for extraction:

• reporting/_json.py — format_json() and JSON-specific helpers
• reporting/_terminal.py — format_terminal(), _colorize(), terminal helpers
• reporting/_markdown.py — format_markdown()
• reporting/_summary.py — format_summary(), format_verbose_header(), format_quality_summary()
• reporting/__init__.py — re-exports public API, determine_exit_code(), write_report(), compute_quality(), CheckQuality

Origin

Flagged during Story 32.4 code review (M1 finding). See _bmad-output/implementation-artifacts/32-4-inline-suppression-comments.md.