refactor(home): let useSessionManagement own six session states

5.4: sessionsCursor / showArchivedSessions / sessionMenuId /
pendingSessionDeletion / sessionDetailLoadingId / sessionFullPrompt move into
use-session-management.ts, which now also computes visibleSessions from the
archived flag it owns. Home reads back only what its own JSX renders.

sessions / activeSessionId / creatingSession / pendingSessionId stay in the
shell: each is read by the bootstrap effect or the consultation run before the
session hook is called. Reasons are listed one by one in the progress record.

useSessionManagement params 41 -> 34, useProfileOnboarding 38 -> 26.

The growth contract tightens to useState 36 / useRef 37 / lines 1846 and gains an
assertion that useSessionManagement still runs before useRectificationSurface,
with the opener handed over through rectificationSessionOpenerRef.

Zero behavior change, zero copy change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JUei7K13cYxLHE3Axe4A45

frontend/src/hooks/use-session-management.ts

@@ -1,7 +1,7 @@
 "use client";
 
 import type { Dispatch, MutableRefObject, SetStateAction } from "react";
-import { useRef } from "react";
+import { useRef, useState } from "react";
 
 import { showChatNotice as setComposerNotice } from "@/lib/chat-notice";
 import { writeChatSession } from "@/lib/chat-session-write-contract";
@@ -12,6 +12,7 @@ import {
   writeSessionUrl,
 } from "@/lib/chat-session-url";
 import { consultationReportMarkdown } from "@/lib/consultation-report-export";
+import { sortSessions } from "@/lib/session-groups";
 import {
   clearBirthTimeConsultationConsent,
   type BirthTimeConsultationConsentState,
@@ -76,15 +77,8 @@ export type SessionManagementParams = {
   setRectificationError: Dispatch<SetStateAction<string>>;
   setRectificationErrorSessionId: Dispatch<SetStateAction<string | null>>;
   setRequestError: Dispatch<SetStateAction<RequestError | null>>;
-  setSessionDetailLoadingId: Dispatch<SetStateAction<string | null>>;
-  setSessionFullPrompt: Dispatch<SetStateAction<{ question: string; theme: Theme } | null>>;
   setSessions: Dispatch<SetStateAction<ChatSession[]>>;
-  sessionsCursor: string | null;
-  setSessionsCursor: Dispatch<SetStateAction<string | null>>;
-  showArchivedSessions: boolean;
-  setShowArchivedSessions: Dispatch<SetStateAction<boolean>>;
   uiPreview: MutableRefObject<boolean>;
-  visibleSessions: ChatSession[];
   openRectificationSession: (exactSessionId: string) => Promise<void> | void;
 };
 
@@ -121,20 +115,27 @@ export function useSessionManagement(params: SessionManagementParams) {
     setRectificationError,
     setRectificationErrorSessionId,
     setRequestError,
-    setSessionDetailLoadingId,
-    setSessionFullPrompt,
     setSessions,
-    sessionsCursor,
-    setSessionsCursor,
-    showArchivedSessions,
-    setShowArchivedSessions,
     uiPreview,
-    visibleSessions,
     openRectificationSession,
   } = params;
+  // Owned here since 2026-09-16 (state lowering batch 2). Home reads back only
+  // what its own JSX renders; the rest never leaves this hook.
+  const [sessionsCursor, setSessionsCursor] = useState<string | null>(null);
+  const [showArchivedSessions, setShowArchivedSessions] = useState(false);
+  const [sessionMenuId, setSessionMenuId] = useState<string | null>(null);
+  const [pendingSessionDeletion, setPendingSessionDeletion] = useState<ChatSession | null>(null);
+  const [sessionDetailLoadingId, setSessionDetailLoadingId] = useState<string | null>(null);
+  const [sessionFullPrompt, setSessionFullPrompt] = useState<{ question: string; theme: Theme } | null>(null);
   sessionsRef.current = sessions;
   const loadMoreInFlight = useRef(false);
 
+  const visibleSessions = sortSessions(sessions.filter((session) => (showArchivedSessions ? session.archivedAt : !session.archivedAt)
+    && (session.sessionType === "birth_time_rectification"
+    || session.messages.length > 0
+    || !session.messagesHydrated
+    || session.id === activeSessionId)));
+
   function updateSession(sessionId: string, change: (session: ChatSession) => ChatSession) {
     setSessions((current) => current.map((session) => (session.id === sessionId ? change(session) : session)));
   }
@@ -520,6 +521,15 @@ export function useSessionManagement(params: SessionManagementParams) {
     }
   }
   return {
+    visibleSessions,
+    sessionsCursor,
+    setSessionsCursor,
+    showArchivedSessions,
+    sessionMenuId,
+    setSessionMenuId,
+    pendingSessionDeletion,
+    setPendingSessionDeletion,
+    setSessionFullPrompt,
     updateSession,
     persistSession,
     ensureSessionMessages,