Minor improvements to databricks-serverless-migration Skill

[alexnastetsky]

Summary

• Added list of common questions for user
• Always generate success report
• Use subagents

Documentation safety checklist

• [ x] Examples use least-privilege permissions (no unnecessary ALL PRIVILEGES, admin tokens, or broad scopes)
• [ x] Elevated permissions are explicitly called out where required
• [ x] Sensitive values are obfuscated (placeholder workspace IDs, URLs, no real tokens)
• [ x] No insecure patterns introduced (e.g. disabled TLS verification, hardcoded credentials)