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/AgentInstance/AgentInstance.tsx

@@ -1,5 +1,6 @@
 import { useMemo } from 'react'
 import { useParams, Link } from 'react-router-dom'
+import { ChevronRight } from 'lucide-react'
 import styles from './AgentInstance.module.css'
 
 // Layout
@@ -177,9 +178,9 @@ export function AgentInstance() {
         {/* Scope trail + badges */}
         <div className={styles.scopeTrail}>
           <Link to="/agents" className={styles.scopeLink}>All Agents</Link>
-          <span className={styles.scopeSep}>&#9656;</span>
+          <span className={styles.scopeSep}><ChevronRight size={12} /></span>
           <Link to={`/agents/${appId}`} className={styles.scopeLink}>{appId}</Link>
-          <span className={styles.scopeSep}>&#9656;</span>
+          <span className={styles.scopeSep}><ChevronRight size={12} /></span>
           <span className={styles.scopeCurrent}>{agent.name}</span>
           <Badge label={agent.status.toUpperCase()} color={statusColor} />
           <Badge label={agent.version} color="auto" variant="outlined" />