fix: lint --format markdown renders report instead of [object Object]

[davideast]

Summary

lint --format markdown emits a literal # [object Object] instead of a readable report. This was originally reported in PR #56, which was closed due to CLA. Thank you to @johansabent for raising it originally.

Before:

$ design.md lint DESIGN.md --format markdown
# [object Object]

After:

$ design.md lint DESIGN.md --format markdown
# Lint Report

**0 errors**, **4 warnings**, **1 infos**

## Findings

- **warning** `components.glass-card-standard`: textColor (#ffffff) on backgroundColor (#ffffff1a) has contrast ratio 1.00:1, below WCAG AA minimum of 4.5:1.
- **info**: Design system defines 47 colors, 6 typography scales, 6 rounding levels, 5 spacing tokens, 10 components.

Root cause

formatAsMarkdown in utils.ts template-literaled obj.summary directly on line 59: \# ${obj.summary}`. This works when summary is a string (the legacy fixer shape) but the lint command returns { errors: number, warnings: number, infos: number }, which coerces to [object Object]`.

Fix

• Detect the lint output shape (findings array + numeric summary object) and route to a dedicated formatLintAsMarkdown renderer
• Gate the legacy obj.summary branch on typeof obj.summary === 'string' so fixer/diff shapes still work
• Render a proper markdown report: heading, severity counts, and a bulleted findings list with optional code-formatted paths

Tests

New utils.test.ts with 6 tests:

• Lint-shape input does not contain [object Object] and produces a headed report (regression test)
• Findings with and without a path render correctly
• Empty findings omit the Findings section
• --format md alias works
• Legacy fixer shape with string summary still renders correctly
• Default format still returns valid JSON

All 244 tests pass.