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/gear.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256"><rect width="256" height="256" fill="none"/><circle cx="128" cy="128" r="40" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/><path d="M41.43,178.09A99.14,99.14,0,0,1,31.36,153.8l16.78-21a81.59,81.59,0,0,1,0-9.64l-16.77-21a99.43,99.43,0,0,1,10.05-24.3l26.71-3a81,81,0,0,1,6.81-6.81l3-26.7A99.14,99.14,0,0,1,102.2,31.36l21,16.78a81.59,81.59,0,0,1,9.64,0l21-16.77a99.43,99.43,0,0,1,24.3,10.05l3,26.71a81,81,0,0,1,6.81,6.81l26.7,3a99.14,99.14,0,0,1,10.07,24.29l-16.78,21a81.59,81.59,0,0,1,0,9.64l16.77,21a99.43,99.43,0,0,1-10,24.3l-26.71,3a81,81,0,0,1-6.81,6.81l-3,26.7a99.14,99.14,0,0,1-24.29,10.07l-21-16.78a81.59,81.59,0,0,1-9.64,0l-21,16.77a99.43,99.43,0,0,1-24.3-10l-3-26.71a81,81,0,0,1-6.81-6.81Z" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/></svg>