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/router.tsx

@@ -1,4 +1,4 @@
-import { createBrowserRouter } from 'react-router-dom'
+import { createBrowserRouter, Navigate, useParams } from 'react-router-dom'
 import * as Sentry from '@sentry/react'
 import { Suspense } from 'react'
 import { AppLayout, ProtectedRoute } from '@/components/layout'
@@ -49,7 +49,10 @@ const ScriptLibraryPage = lazyWithRetry(() => import('@/pages/ScriptLibraryPage'
 const ScriptManagePage = lazyWithRetry(() => import('@/pages/ScriptManagePage'))
 const AssistantChatPage = lazyWithRetry(() => import('@/pages/AssistantChatPage'))
 const FlowAssistPage = lazyWithRetry(() => import('@/pages/FlowAssistPage'))
-const FlowPilotSessionPage = lazyWithRetry(() => import('@/pages/FlowPilotSessionPage'))
+// FlowPilotSessionPage (the old guided-mode surface) was removed from active
+// routing in Phase 1 of the FlowPilot migration. The file is retained on disk
+// for reference but not mounted — the unified chat-primary surface now serves
+// /pilot. Delete the file when nothing in the tree references it anymore.
 const EscalationQueuePage = lazyWithRetry(() => import('@/pages/EscalationQueuePage'))
 const ReviewQueuePage = lazyWithRetry(() => import('@/pages/ReviewQueuePage'))
 const FlowPilotAnalyticsPage = lazyWithRetry(() => import('@/pages/FlowPilotAnalyticsPage'))
@@ -98,6 +101,16 @@ function page(Component: React.LazyExoticComponent<React.ComponentType>) {
   )
 }
 
+/**
+ * Permanent 301-style redirect from /assistant/:sessionId to /pilot/:sessionId.
+ * Used by the Phase 1 route-rename; paired with a bare-path redirect to /pilot.
+ * SPA redirects replace history so the legacy URL does not linger in back-nav.
+ */
+function AssistantSessionRedirect() {
+  const { sessionId } = useParams<{ sessionId: string }>()
+  return <Navigate to={sessionId ? `/pilot/${sessionId}` : '/pilot'} replace />
+}
+
 export const router = sentryCreateBrowserRouter([
   {
     path: '/landing',
@@ -202,11 +215,14 @@ export const router = sentryCreateBrowserRouter([
       { path: 'network-diagrams/new', element: page(DiagramEditorPage) },
       { path: 'network-diagrams/:id', element: page(DiagramEditorPage) },
       { path: 'kb-accelerator', element: page(KBAcceleratorPage) },
-      { path: 'assistant', element: page(AssistantChatPage) },
-      { path: 'assistant/:sessionId', element: page(AssistantChatPage) },
+      // Phase 1 — FlowPilot migration. The unified chat-primary surface lives at
+      // /pilot; /assistant permanently redirects. FlowPilotSessionPage (old
+      // guided surface) is no longer mounted.
+      { path: 'pilot', element: page(AssistantChatPage) },
+      { path: 'pilot/:sessionId', element: page(AssistantChatPage) },
+      { path: 'assistant', element: <Navigate to="/pilot" replace /> },
+      { path: 'assistant/:sessionId', element: <AssistantSessionRedirect /> },
       { path: 'flow-assist', element: page(FlowAssistPage) },
-      { path: 'pilot', element: page(FlowPilotSessionPage) },
-      { path: 'pilot/:sessionId', element: page(FlowPilotSessionPage) },
       { path: 'escalations', element: page(EscalationQueuePage) },
       { path: 'queue', element: page(SessionQueuePage) },
       { path: 'review-queue', element: page(ReviewQueuePage) },