feat: $B ux-audit command + snapshot --heatmap flag

New browse meta-command: ux-audit extracts page structure (site ID, navigation,
headings, interactive elements, text blocks) as structured JSON for agent-side
UX behavioral analysis. Pure data extraction — the agent applies the 6 usability
tests and makes judgment calls. Element caps: 50 headings, 100 links, 200
interactive, 50 text blocks.

New snapshot flag: -H/--heatmap accepts a JSON color map mapping ref IDs to
colors (green/yellow/red/blue/orange/gray). Extends existing snapshot -a
annotation system with per-ref colors instead of hardcoded red. Color whitelist
validation prevents CSS injection. Composable — any skill can use it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/skill-validation.test.ts

@@ -143,6 +143,7 @@ describe('Command registry consistency', () => {
     const validKeys = new Set([
       'interactive', 'compact', 'depth', 'selector',
       'diff', 'annotate', 'outputPath', 'cursorInteractive',
+      'heatmap',
     ]);
     for (const flag of SNAPSHOT_FLAGS) {
       expect(validKeys.has(flag.optionKey)).toBe(true);