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/flowpilot/AISessionListItem.tsx

@@ -18,9 +18,12 @@ const STATUS_CONFIG = {
 export function AISessionListItem({ session }: AISessionListItemProps) {
   const config = STATUS_CONFIG[session.status as keyof typeof STATUS_CONFIG] ?? STATUS_CONFIG.active
   const StatusIcon = config.icon
+  // Both chat and guided sessions now land on the unified /pilot surface.
+  // session_type is preserved on the DB row for data compatibility but is
+  // no longer used for frontend route selection (Phase 1 FlowPilot migration).
   const isChat = session.session_type === 'chat'
   const TypeIcon = isChat ? MessageCircle : Route
-  const linkTo = isChat ? `/assistant/${session.id}` : `/pilot/${session.id}`
+  const linkTo = `/pilot/${session.id}`
   const displayTitle = isChat
     ? (session.title || session.problem_summary || 'Untitled chat')
     : (session.problem_summary || 'Untitled session')