feat(pilot): unify AI troubleshooting surface at /pilot, redirect /assistant (Phase 1)

Collapses the pre-existing dual-surface setup (AssistantChatPage at /assistant,
FlowPilotSessionPage at /pilot) into a single chat-primary surface per
architectural claim #1 of FLOWPILOT-MIGRATION.md.

Router changes (frontend/src/router.tsx):
- /pilot and /pilot/:sessionId now render AssistantChatPage.
- /assistant redirects permanently to /pilot via <Navigate replace>.
- /assistant/:sessionId redirects to /pilot/:sessionId preserving the ID
  via an AssistantSessionRedirect helper that reads the param.
- FlowPilotSessionPage is no longer imported or mounted. Per the
  beta-history-disposable decision, the file stays on disk for reference
  but is unreachable; delete once nothing else in the tree imports it.

Dispatcher de-branching — previously these sites routed by session_type
(chat -> /assistant, otherwise -> /pilot). All now unconditionally go to
/pilot/:id since session_type is no longer used for frontend routing:
- components/dashboard/ActiveFlowPilotSessions.tsx
- components/dashboard/RecentFlowPilotSessions.tsx
- components/flowpilot/AISessionListItem.tsx
  (keeps isChat for icon selection, but linkTo is unconditional)

User-facing label + navigation updates:
- components/layout/CommandPalette.tsx: "AI Assistant" palette entry
  becomes "FlowPilot" pointing to /pilot; the sparkles quick-action also
  routes to /pilot.
- components/dashboard/StartSessionInput.tsx: both navigate() call sites
  now go to /pilot instead of /assistant.
- lib/routePrefetch.ts: prefetch entry for AssistantChatPage keyed to
  /pilot (the real surface) rather than /assistant (now redirect-only).

Preserved intentionally (not user-facing routes):
- Backend /assistant/retention API path and the assistantChatApi module
  name — those are internal API and module identifiers, not SPA routes.
- src/components/assistant/* and src/types/assistant-chat — TypeScript
  module paths, not routes.
- Sidebar.tsx — no top-level AI entry existed to rename; /pilot is
  already in the History group's matchPaths. Whether FlowPilot deserves
  its own rail entry is a future UX decision, not Phase 1 scope.
- FlowPilotAnalyticsPage at /analytics/flowpilot — analytics for the
  unified product, not guided-only, per the agreed Q16 interpretation.

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

frontend/src/components/layout/CommandPalette.tsx

@@ -42,7 +42,7 @@ const PAGES: PaletteItem[] = [
   { id: 'page-dashboard', group: 'pages', title: 'Dashboard', path: '/', icon: 'page' },
   { id: 'page-flows', group: 'pages', title: 'All Flows', subtitle: 'Browse your flow library', path: '/trees', icon: 'page' },
   { id: 'page-sessions', group: 'pages', title: 'Sessions', subtitle: 'View session history', path: '/sessions', icon: 'page' },
-  { id: 'page-assistant', group: 'pages', title: 'AI Assistant', subtitle: 'FlowPilot chat', path: '/assistant', icon: 'page' },
+  { id: 'page-flowpilot', group: 'pages', title: 'FlowPilot', subtitle: 'AI troubleshooting', path: '/pilot', icon: 'page' },
   { id: 'page-scripts', group: 'pages', title: 'Script Generator', subtitle: 'Generate PowerShell scripts', path: '/scripts', icon: 'page' },
   { id: 'page-analytics', group: 'pages', title: 'Analytics', subtitle: 'Team usage & metrics', path: '/analytics', icon: 'page' },
   { id: 'page-settings', group: 'pages', title: 'Settings', subtitle: 'Account & preferences', path: '/account', icon: 'page' },
@@ -177,7 +177,7 @@ export function CommandPalette({ open, onClose }: CommandPaletteProps) {
       group: 'flowpilot',
       title: 'Troubleshoot with FlowPilot',
       subtitle: trimmed,
-      path: '/assistant',
+      path: '/pilot',
       icon: 'sparkles',
     }