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/actions/configureSparks.ts

@@ -16,10 +16,10 @@ const inputSpec = InputSpec.of({
   spark1_user: Value.text({
     name: 'Spark 1 SSH user',
     description:
-      'The user account on Spark 1 to SSH in as. DGX Sparks ship with "modelo" as the default user — change only if you customized yours.',
+      'The user account on Spark 1 to SSH in as — whatever you log in as when you ssh into it manually.',
     required: true,
-    default: 'modelo',
-    placeholder: 'modelo',
+    default: null,
+    placeholder: 'your SSH username',
     masked: false,
   }),
   spark2_host: Value.text({
@@ -36,8 +36,8 @@ const inputSpec = InputSpec.of({
     description:
       'The user account on Spark 2 to SSH in as. Usually the same as Spark 1.',
     required: true,
-    default: 'modelo',
-    placeholder: 'modelo',
+    default: null,
+    placeholder: 'your SSH username',
     masked: false,
   }),
 })