Remember the New Workspace isolation choice

The local/worktree choice was unstored component state and reset on every
remount of the New Workspace screen (e.g. after creating a workspace).
Persist it in the create-form preferences (FormPreferences.isolation).

Also rename the app-layer "backing" to "isolation" to match its UI label.
Isolation is the create-time intent; it stays distinct from workspaceKind,
the git-derived persisted property, and the wire's source.kind.

packages/app/src/create-agent-preferences/preferences.ts

@@ -33,6 +33,7 @@ const formPreferencesSchema = z.object({
       }),
     )
     .optional(),
+  isolation: z.enum(["local", "worktree"]).optional(),
 });
 
 export type ProviderPreferences = z.infer<typeof providerPreferencesSchema>;