0.1.0:2 - remove '<spark-user>' default everywhere (it's Alice's username, not factory)

Per user correction: '<spark-user>' 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 '<spark-user>' fallback; skip Spark if user not configured
- Update feedback memory with the correction

package/startos/fileModels/sparkConfig.yaml.ts

@@ -4,9 +4,9 @@ import { sdk } from '../sdk'
 
 export const sparkConfigSchema = z.object({
   spark1_host: z.string().catch(''),
-  spark1_user: z.string().catch('<spark-user>'),
+  spark1_user: z.string().catch(''),
   spark2_host: z.string().catch(''),
-  spark2_user: z.string().catch('<spark-user>'),
+  spark2_user: z.string().catch(''),
 })
 
 export type SparkConfig = z.infer<typeof sparkConfigSchema>