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/dashboard/ActiveFlowPilotSessions.tsx

@@ -48,7 +48,7 @@ export function ActiveFlowPilotSessions({ hideHeader = false }: { hideHeader?: b
           {sessions.map((session) => (
             <button
               key={session.id}
-              onClick={() => navigate(session.session_type === 'chat' ? `/assistant/${session.id}` : `/pilot/${session.id}`)}
+              onClick={() => navigate(`/pilot/${session.id}`)}
               className="card-interactive p-4 text-left"
             >
               <div className="flex items-start justify-between gap-2 mb-2">

frontend/src/components/dashboard/RecentFlowPilotSessions.tsx

@@ -52,7 +52,7 @@ export function RecentFlowPilotSessions({ hideHeader = false }: { hideHeader?: b
           return (
             <button
               key={session.id}
-              onClick={() => navigate(session.session_type === 'chat' ? `/assistant/${session.id}` : `/pilot/${session.id}`)}
+              onClick={() => navigate(`/pilot/${session.id}`)}
               className="flex w-full items-center gap-3 px-5 py-3 text-left hover:bg-[rgba(255,255,255,0.02)] transition-colors"
               style={{
                 borderBottom: i < sessions.length - 1 ? '1px solid var(--color-border-default)' : undefined,

frontend/src/components/dashboard/StartSessionInput.tsx

@@ -52,7 +52,7 @@ export function StartSessionInput() {
     if (completedUploadIds.length > 0) {
       state.uploadIds = completedUploadIds
     }
-    navigate('/assistant', { state })
+    navigate('/pilot', { state })
   }
 
   const handleKeyDown = (e: React.KeyboardEvent) => {
@@ -63,7 +63,7 @@ export function StartSessionInput() {
   }
 
   const handleSuggestionClick = (suggestion: string) => {
-    navigate('/assistant', { state: { prefill: suggestion } })
+    navigate('/pilot', { state: { prefill: suggestion } })
   }
 
   // ── File handling ──────────────────────────────