feat(frontend): sprint 3 — multi-technique MITRE selection + matrix modal

- types: replace mitre_technique_id/name scalars with techniques:MitreTechnique[]
  on Simulation; add MitreTactic/MitreMatrixTechnique/MitreMatrixSubtechnique;
  SimulationPatchInput now uses technique_ids:string[]
- api/mitre.ts: add getMitreMatrix() → GET /api/mitre/matrix
- hooks/useMitre: add useMitreMatrix(enabled) with staleTime:Infinity
- MitreTechniquePicker: clean rewrite — onSelect(technique) one-shot, resets
  input after selection, no incoming value props
- MitreTechniqueTag: chip component with id+name and × remove button
- MitreMatrixModal: tactic columns (220px fixed), expand/collapse subtechniques,
  search filter (auto-expands parent on sub match), selection state, focus trap
  (Tab wrap, Escape, search autofocus), backdrop click cancel, Apply N techniques
- MitreTechniquesField: orchestrates tags+picker+matrix with auto-save PATCH on
  every add/remove/Apply, dedup guard, disabled read-only mode for SOC
- SimulationFormPage: swap MitreTechniquePicker for MitreTechniquesField; remove
  technique state from RT form (techniques have independent auto-save cycle)
- SimulationList: MITRE column → T1059 +2 counter format, — when empty
- Tests: 84 passing (13 test files); new suites for Tag, Field, Modal;
  MitreTechniquePicker + SimulationFormPage + SimulationList adapted to new API

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/SimulationFormPage.tsx

@@ -16,12 +16,10 @@ import { LoadingState } from '@/components/LoadingState';
 import { ErrorState } from '@/components/ErrorState';
 import { SimulationStatusBadge } from '@/components/SimulationStatusBadge';
 import { ConfirmDialog } from '@/components/ConfirmDialog';
-import { MitreTechniquePicker } from '@/components/MitreTechniquePicker';
+import { MitreTechniquesField } from '@/components/MitreTechniquesField';
 
 interface RedteamFormState {
   name: string;
-  mitre_technique_id: string | null;
-  mitre_technique_name: string | null;
   description: string;
   commands: string;
   prerequisites: string;
@@ -38,8 +36,6 @@ interface SocFormState {
 
 const EMPTY_RT: RedteamFormState = {
   name: '',
-  mitre_technique_id: null,
-  mitre_technique_name: null,
   description: '',
   commands: '',
   prerequisites: '',
@@ -81,8 +77,6 @@ export function SimulationFormPage(): JSX.Element {
       const s = detail.data;
       setRt({
         name: s.name,
-        mitre_technique_id: s.mitre_technique_id,
-        mitre_technique_name: s.mitre_technique_name,
         description: s.description ?? '',
         commands: s.commands ?? '',
         prerequisites: s.prerequisites ?? '',
@@ -154,8 +148,6 @@ export function SimulationFormPage(): JSX.Element {
     }
     const patch: SimulationPatchInput = {
       name: rt.name.trim(),
-      mitre_technique_id: rt.mitre_technique_id ?? null,
-      mitre_technique_name: rt.mitre_technique_name ?? null,
       description: rt.description.trim() || null,
       commands: rt.commands.trim() || null,
       prerequisites: rt.prerequisites.trim() || null,
@@ -314,16 +306,15 @@ export function SimulationFormPage(): JSX.Element {
           />
         </FormField>
 
-        <FormField label="MITRE Technique" htmlFor="sim-mitre">
-          <MitreTechniquePicker
-            techniqueId={rt.mitre_technique_id}
-            techniqueName={rt.mitre_technique_name}
-            onChange={(id, name) =>
-              setRt({ ...rt, mitre_technique_id: id, mitre_technique_name: name })
-            }
+        <div className="flex flex-col gap-xs">
+          <span className="text-[14px] font-medium text-ink">MITRE Techniques</span>
+          <MitreTechniquesField
+            value={simulation?.techniques ?? []}
+            simulationId={simulationId as number}
+            engagementId={engagementId as number}
             disabled={rtDisabled}
           />
-        </FormField>
+        </div>
 
         <FormField label="Description" htmlFor="sim-description">
           <TextArea