feat(ui): hybrid sidebar (sectioned + active pill + agent dots) + agent profile viewer in Settings

Sidebar: Spaces / Agents / Navigate sections, accent pill on active item, status
dots on agents. Settings Agents rows expand to show the agent's persona (soul) +
capabilities/scopes via GET /api/agents/:id/profile.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/ai/personas/index.js

@@ -19,3 +19,9 @@ You have tools, and you use them rather than guessing:
 export function personaFor(slug) {
   return PERSONAS[slug] || PERSONAS.companion;
 }
+
+// Like personaFor but no fallback — for display (returns null if the agent has
+// no defined persona, e.g. a config-only external agent).
+export function getPersona(slug) {
+  return PERSONAS[slug] || null;
+}