feat(cortex): Epic 2 - @Mention Systeem (E2.S1-S3)

Inline patient selectie via @naam in de chat input.

YAGNI Versimpeling: 8→4 SP, 5→3 stories
- Geen selectedMentions array (activePatient is al beschikbaar)
- Geen API payload wijziging nodig
- Geen meerdere mentions tracking

E2.S1 - @-detectie:
- Detecteer laatste @ in input
- Track query en startIndex
- Actief als geen space na @ en minimaal 1 char

E2.S2 - PatientMentionDropdown:
- Hergebruik usePatientSearch (debounce 150ms, limit 5)
- Hergebruik PatientListItem (size="sm")
- Positioned above input (bottom-full)
- Escape sluit dropdown

E2.S3 - Selectie flow:
- Vervang @query met @naam in input
- Zet activePatient via usePatientSelection
- Sluit dropdown automatisch

Bouwplan v1.4: Epic 2 compleet (14/19 SP - 74%)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

components/cortex/command-center/patient-mention-dropdown.tsx

@@ -0,0 +1,90 @@
+'use client';
+
+/**
+ * PatientMentionDropdown Component
+ *
+ * Dropdown for @mention patient selection in CommandInput.
+ * Positioned above the input, shows search results.
+ *
+ * Epic: E2.S2 (Patient Selectie - @Mention)
+ */
+
+import { useEffect } from 'react';
+import { usePatientSearch, type PatientSearchResult } from '@/lib/cortex/hooks/use-patient-search';
+import {
+  PatientListItem,
+  PatientListEmpty,
+  PatientListLoading,
+} from '@/components/cortex/shared/patient-list-item';
+
+interface PatientMentionDropdownProps {
+  /** Current search query (text after @) */
+  query: string;
+  /** Called when a patient is selected */
+  onSelect: (patient: PatientSearchResult) => void;
+  /** Called when dropdown should close */
+  onClose: () => void;
+}
+
+export function PatientMentionDropdown({
+  query,
+  onSelect,
+  onClose,
+}: PatientMentionDropdownProps) {
+  const { setQuery, results, isSearching } = usePatientSearch({
+    debounceMs: 150,
+    limit: 5,
+    minQueryLength: 2,
+  });
+
+  // Sync query to search hook
+  useEffect(() => {
+    setQuery(query);
+  }, [query, setQuery]);
+
+  // Close on Escape
+  useEffect(() => {
+    const handleKeyDown = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') {
+        e.preventDefault();
+        e.stopPropagation();
+        onClose();
+      }
+    };
+
+    document.addEventListener('keydown', handleKeyDown);
+    return () => document.removeEventListener('keydown', handleKeyDown);
+  }, [onClose]);
+
+  return (
+    <div
+      className="absolute bottom-full left-0 mb-2 w-72 bg-white border border-slate-200 rounded-lg shadow-lg z-50 overflow-hidden"
+      role="listbox"
+      aria-label="Patient zoekresultaten"
+    >
+      {isSearching ? (
+        <PatientListLoading message="Zoeken..." />
+      ) : query.length < 2 ? (
+        <div className="p-3 text-sm text-slate-500 text-center">
+          Typ minimaal 2 karakters
+        </div>
+      ) : results.length > 0 ? (
+        <div className="p-1 space-y-0.5 max-h-64 overflow-y-auto">
+          {results.map((patient) => (
+            <PatientListItem
+              key={patient.id}
+              patient={patient}
+              onClick={() => onSelect(patient)}
+              size="sm"
+            />
+          ))}
+        </div>
+      ) : (
+        <PatientListEmpty
+          message={`Geen resultaten voor "@${query}"`}
+          submessage="Probeer een andere naam"
+        />
+      )}
+    </div>
+  );
+}