0.1.0:2 - remove 'modelo' default everywhere (it's Grant's username, not factory)

Per user correction: 'modelo' is not the DGX Spark factory default. Generic-ize:
- configureSparks: no default user; placeholder 'your SSH username'
- sparkConfig schema: empty string defaults
- main.ts env fallback: empty
- showPublicKey: drop the 'modelo' fallback; skip Spark if user not configured
- Update feedback memory with the correction

package/startos/main.ts

@@ -10,9 +10,9 @@ export const main = sdk.setupMain(async ({ effects }) => {
   // Changing this file via the "Configure Sparks" action restarts the daemon.
   const cfg = (await sparkConfigYaml.read().const(effects)) ?? {
     spark1_host: '',
-    spark1_user: 'modelo',
+    spark1_user: '',
     spark2_host: '',
-    spark2_user: 'modelo',
+    spark2_user: '',
   }
 
   return sdk.Daemons.of(effects).addDaemon('primary', {