feat(m6): missions + snapshot CRUD, membership visibility, status state machine

Adds the mission layer that materialises template snapshots, plus the SPA
list / 3-step wizard / detail page.

Backend:
- app/services/missions.py — create_mission snapshots scenarios, tests, MITRE
  tags in a 4-query write; list/get apply a non-admin membership filter that
  collapses to 404 (no existence leak); status state machine enforces
  draft → in_progress → completed → archived with archived as a sink; the
  non-admin creator is auto-added as role_hint='red' to retain visibility.
- app/api/missions.py — 8 endpoints (list, get, create, update, add
  scenarios, set members, transition, soft-delete) with strict pydantic
  schemas. The transition endpoint splits the perm gate manually so
  archive requires mission.archive while other targets use mission.update.
- app/api/users.py — new GET /users/roster returning (id, email,
  display_name) only, gated by user.read OR mission.create OR
  mission.update — lets non-admin wizard users see assignable peers
  without exposing the admin /users payload.
- app/api/diag.py — /diag/reset truncates the mission_* tables before the
  template tables because the source_*_template_id FKs are ON DELETE SET
  NULL, which is cheaper to short-circuit by removing the children first.

Frontend:
- lib/missions.ts — typed client, queryKey factory, status accent map.
- pages/MissionsListPage.tsx — list cards with status accent + filters
  (q, client, status).
- pages/MissionsCreatePage.tsx — 3-step wizard (meta → scenarios → members)
  with member roster fed by /users/roster.
- pages/MissionDetailPage.tsx — header + transition buttons (legal next
  states only) + Tests/Members/Synthesis/Export tabs.
- Routes + nav entry (visible to anyone with mission.read or admin).

Tests:
- backend/tests/test_missions.py — 22 pytest covering snapshot fidelity,
  MITRE propagation, membership visibility, transition state machine,
  perm gating, member set replace, append scenarios, soft-delete, partial
  update, inverted-date rejection.
- e2e/tests/m6-missions.spec.ts — 5 Playwright (snapshot freezing, non-admin
  visibility, status transitions + 409, SPA wizard end-to-end, list filter).

Docs:
- CHANGELOG, tasks/testing-m6.md, tasks/lessons.md (snapshot tradeoffs,
  membership=404 pattern, /diag/reset order, auto-creator add).
- README + tasks/todo.md updated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -37,6 +37,9 @@ export function Layout() {
                 {navItem('/', 'Home')}
                 {navItem('/profile', 'Profile')}
                 {navItem('/mitre', 'MITRE')}
+                {(state.user.is_admin ||
+                  state.user.permissions.includes('mission.read')) &&
+                  navItem('/missions', 'Missions')}
                 {state.user.is_admin && (
                   <>
                     {navItem('/admin/users', 'Users')}
@@ -71,7 +74,7 @@ export function Layout() {
         <Outlet />
 
         <footer className="mt-[60px] py-8 border-t border-border text-center font-mono text-xs text-text-dim">
-          metamorph · M0 bootstrap · M1 db schema · M2 auth · M3 rbac · M4 mitre · M5 templates · design system from tasks/design.md
+          metamorph · M0 bootstrap · M1 db schema · M2 auth · M3 rbac · M4 mitre · M5 templates · M6 missions · design system from tasks/design.md
         </footer>
       </div>
     </div>