feat(icons): adopt Phosphor as the icon system, ship curated 80-icon set

- Restructure assets/icons: drop ui/, rename outline->regular, filled->fill, add bold/
- Pull 80 Phosphor icons in regular weight (all) + fill weight (all) + bold (status set)
- Include Phosphor MIT LICENSE
- Rewrite icons README with folder map, specs, usage examples, contribution rules

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

assets/icons/regular/arrow-square-out.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><polyline points="216 104 215.99 40.01 152 40" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/><line x1="136" y1="120" x2="216" y2="40" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/><path d="M184,136v72a8,8,0,0,1-8,8H48a8,8,0,0,1-8-8V80a8,8,0,0,1,8-8h72" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/></svg>