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/users.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="84" cy="108" r="52" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/><path d="M10.23,200a88,88,0,0,1,147.54,0" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/><path d="M172,160a87.93,87.93,0,0,1,73.77,40" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/><path d="M152.69,59.7A52,52,0,1,1,172,160" fill="none" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="16"/></svg>