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.

src/server/index.ts

@@ -11,7 +11,6 @@ export * from "./commit-positions.ts";
 export * from "./jobs.ts";
 export * from "./stores.ts";
 export { readRenderSettings, writeRenderSettings } from "../store/render-settings.ts";
-export { readDefaultSettings, writeDefaultSettings } from "../store/default-settings.ts";
 // The settings record + prompt overrides (settings-storage wave, 2026-09-19).
 // The *Record aliases keep the app/api/settings routes explicit about which
 // seam they touch (the raw record, not the chat-section convenience view).

src/server/model-catalog.ts

@@ -39,20 +39,24 @@ import { BACKGROUND_MODEL_ROLES, MODEL_SLOTS, type BackgroundModelRole, type Mod
 /** The picker's client-safe model entry (contract: core/contracts/models.ts). */
 export type ModelDto = ModelCatalogEntry;
 
-/** The conversation agents' resolved default (pure form — the record's chat
- *  section + the strong slot in, CatalogDefaults out; `catalogDefaults()`
- *  below is the fs wrapper). BYOK (owner, 2026-09-22): there is no factory
- *  floor — when neither the chat section nor the strong slot names a
- *  model, the default is NULL and the turn gate refuses with the 503
- *  models_not_configured envelope. An unset thinkingLevel resolves from
- *  DEFAULT_THINKING_ORDER at engine build (reported as absent here). */
+/**
+ * The conversation agents' resolved default (pure form — the strong slot
+ * in, CatalogDefaults out; `catalogDefaults()` below is the fs wrapper).
+ * PER-SESSION MODELS (owner, 2026-09-28): every session owns its model +
+ * thinking level from birth, stamped verbatim from the strong slot — the
+ * STRONG SLOT IS THE NEW-SESSION DEFAULT, and the resolve-time fallback
+ * for legacy settings-less records. BYOK (owner, 2026-09-22): there is no
+ * factory floor — when the strong slot names no model, the default is
+ * NULL and the turn gate refuses with the 503 models_not_configured
+ * envelope. An unset thinkingLevel resolves from DEFAULT_THINKING_ORDER at
+ * engine build (reported as absent here).
+ */
 export function catalogDefaultsFor(
-	chat: ChatSettings | undefined,
 	strong: EffectiveSlot | null,
 ): CatalogDefaults | null {
-	const model = chat?.model ?? (strong ? { provider: strong.provider, id: strong.id } : undefined);
+	const model = strong ? { provider: strong.provider, id: strong.id } : undefined;
 	if (!model) return null;
-	const level = chat?.thinkingLevel ?? strong?.thinkingLevel;
+	const level = strong?.thinkingLevel;
 	return {
 		model,
 		...(level !== undefined ? { thinkingLevel: level } : {}),
@@ -60,11 +64,9 @@ export function catalogDefaultsFor(
 }
 
 /** The conversation agents' resolved default — what a settings-less chat
- *  runs. (settings-storage wave, 2026-09-19): the user's picked default
- *  rides in the settings record's `chat` section; the picker's defaults
- *  row shows it once chosen. */
+ *  runs: the strong slot, i.e. the birth-default for new sessions. */
 export function catalogDefaults(): CatalogDefaults | null {
-	return catalogDefaultsFor(settingsStore.readSettings()?.chat, effectiveSettings().models.strong);
+	return catalogDefaultsFor(effectiveSettings().models.strong);
 }
 
 /** Project the resolved per-role specs to the client DTO (pure — the