per-session models: every session owns its model + thinking level from birth

The reported bug (2026-09-28): every chat ran glm-5.3-flash regardless of
any pick, while composer and settings UIs agreed with each other and lied
about the run. Root cause: a fossil default-settings record (settings.json
chat section, frozen pre-slots-wave) was stamped verbatim into every new
chat at birth by the new-session flow, and resolveConversationSpec prefers
the per-chat record over the strong slot forever.

Owner redesign — models are per session, always; the strong slot is the
new-session default:

- POST /api/chats stamps the newborn verbatim from the strong slot
  (model + thinking level); no client settings PUT on create anymore
- the composer control is dual-bound: in-session it reads/writes the
  session's own record (GET /api/chats/:id, PUT .../settings); on the
  draft surfaces it binds the strong slot (picking there = changing the
  default)
- thinking levels follow the identical mechanics (confirmed by owner)
- the zombie default-settings system dies whole: useDefaultSettings,
  /api/settings/defaults, src/store/default-settings.ts, the chat
  section of UserSettings (sanitizer strips fossil copies), the legacy
  default-settings.json fold, and DEFAULT_SETTINGS_FILE
- catalogDefaultsFor collapses to the strong slot alone
- thinking display for a legacy unstamped session resolves exactly like
  the engine (session ?? strong ?? model ladder)

Tests: new chat-birth-stamp suite pins the stamp, the unconfigured boot,
the unset level, and the copy-not-live-view regression; defaults pins
rewritten strong-only; legacy-migration suite deleted.

components/home.tsx

@@ -6,11 +6,10 @@ import { CornerDownLeftIcon } from "lucide-react";
 import { GraphView } from "@/components/graph-view";
 import { SetupDialog } from "@/components/setup-dialog";
 import { useStrongKeyless, useZeroKeys, ZeroKeysPill } from "@/components/setup-cta";
-import { useDefaultSettings } from "@/components/default-settings";
 import { DictationBar, useDictation } from "@/components/dictation";
 import { SidebarToggle } from "@/components/sidebar";
 import { StatsBar } from "@/components/stats-bar";
-import type { ChatSettings, TurnUsage } from "@/src/core";
+import type { TurnUsage } from "@/src/core";
 
 /**
  * Home — the knowledge graph is the main view, full-bleed edge to edge
@@ -18,7 +17,7 @@ import type { ChatSettings, TurnUsage } from "@/src/core";
  * under the header and the new-session dock, both of which float over it
  * with no gradient scrims. Sessions live in the toggleable sidebar.
  */
-export function Home({ initialSettings, initialConfigured }: { initialSettings: ChatSettings | null; initialConfigured: boolean }) {
+export function Home({ initialConfigured }: { initialConfigured: boolean }) {
 	const router = useRouter();
 	const [topic, setTopic] = useState("");
 	// First-run gate (model-slots wave, owner 2026-09-20): chats cannot
@@ -41,13 +40,6 @@ export function Home({ initialSettings, initialConfigured }: { initialSettings:
 		append: (t) => setTopic((v) => (v ? v.replace(/\s+$/, "") + " " + t : t)),
 	});
 	const [busy, setBusy] = useState(false);
-	// THE default settings (owner, 2026-09-18): this topic dock IS the
-	// settings page for the default model — no baked-in default exists; the
-	// user's last choice (server-rendered in via initialSettings, shared
-	// with /chat/new through data/default-settings.json) IS the default.
-	// Still carried to the chat POST then PUT-on-create (the model picker
-	// defers its save when chatId is empty; see components/model-picker).
-	const { settings, save: saveDraft } = useDefaultSettings(initialSettings);
 
 	const start = async (force = false) => {
 		const t = topic.trim();
@@ -79,14 +71,9 @@ export function Home({ initialSettings, initialConfigured }: { initialSettings:
 				body: JSON.stringify({ title: t }),
 			});
 			const { id } = (await res.json()) as { id: string };
-			if (settings) {
-				// Best-effort persist — the selection was made pre-chat.
-				await fetch(`/api/chats/${id}/settings`, {
-					method: "PUT",
-					headers: { "Content-Type": "application/json" },
-					body: JSON.stringify(settings),
-				}).catch(() => {});
-			}
+			// No settings PUT on create (per-session models, owner
+			// 2026-09-28): the SERVER stamps the newborn from the strong
+			// slot — the same default this surface's model control edits.
 			router.push(`/chat/${id}?prompt=${encodeURIComponent(t)}`);
 		} catch {
 			setBusy(false);