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>

browse/SKILL.md

@@ -587,6 +587,7 @@ The snapshot is your primary tool for understanding and interacting with pages.
 -a        --annotate              Annotated screenshot with red overlay boxes and ref labels
 -o <path> --output                Output path for annotated screenshot (default: <temp>/browse-annotated.png)
 -C        --cursor-interactive    Cursor-interactive elements (@c refs — divs with pointer, onclick). Auto-enabled when -i is used.
+-H <json> --heatmap               Color-coded overlay screenshot from JSON map: '{"@e1":"green","@e3":"red"}'. Valid colors: green, yellow, red, blue, orange, gray.
 ```
 
 All flags can be combined freely. `-o` only applies when `-a` is also used.
@@ -717,6 +718,7 @@ $B prettyscreenshot --cleanup --scroll-to ".pricing" --width 1440 ~/Desktop/hero
 | `network [--clear]` | Network requests |
 | `perf` | Page load timings |
 | `storage [set k v]` | Read all localStorage + sessionStorage as JSON, or set <key> <value> to write localStorage |
+| `ux-audit` | Extract page structure for UX behavioral analysis — site ID, nav, headings, text blocks, interactive elements. Returns JSON for agent interpretation. |
 
 ### Visual
 | Command | Description |