feat: migrate all icons to Lucide React

Replace unicode characters, emoji, and inline SVGs with lucide-react
components across the entire design system and page layer. Update
tests to assert on SVG elements instead of text content.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/pages/AgentHealth/AgentHealth.tsx

@@ -1,5 +1,6 @@
 import { useState, useMemo } from 'react'
 import { useParams, Link } from 'react-router-dom'
+import { ChevronRight } from 'lucide-react'
 import styles from './AgentHealth.module.css'
 
 // Layout
@@ -389,7 +390,7 @@ export function AgentHealth() {
           {scope.level !== 'all' && (
             <>
               <Link to="/agents" className={styles.scopeLink}>All Agents</Link>
-              <span className={styles.scopeSep}>&#9656;</span>
+              <span className={styles.scopeSep}><ChevronRight size={12} /></span>
               <span className={styles.scopeCurrent}>{scope.appId}</span>
             </>
           )}