Remove updatedAt from new project config writes

[codeaholicguy]

Summary

• Remove updatedAt from the project config model and newly-created .ai-devkit.json files
• Stop updating or adding updatedAt during ConfigManager.update
• Keep no-op config updates from rewriting files; existing legacy updatedAt values are left alone when there is no real config change
• Update config tests, e2e fixtures, tracked project config, and configuration docs to omit updatedAt for new config examples