Add per-model descriptions + repo-cleanup polish

- models.yaml: add 'description' field for all 5 models (generic, anyone-can-use)
- ModelDef gains optional description: str | None field
- UI: render description below meta tags; mute the repo line further
- escapeHtml() for safety in case descriptions/names contain HTML chars
- Update runbook: how to add a new model with description

image/app/static/style.css

@@ -170,7 +170,16 @@ main {
 }
 .card .name { font-weight: 600; font-size: 15px; }
 .card .meta { display: flex; flex-wrap: wrap; gap: 6px; font-size: 12px; color: var(--muted); }
-.card .repo { word-break: break-all; }
+.card .desc {
+  font-size: 13.5px;
+  line-height: 1.5;
+  color: #b9b9c4;
+}
+.card .repo {
+  word-break: break-all;
+  font-size: 11px;
+  color: #5c5c66;
+}
 .tag {
   background: var(--surface-2);
   border: 1px solid var(--border);