Clean up schema by simplifying version structure (#379)

## Summary

Simplified the schema structure by:
- Replacing `version_detail.version` with direct `version` field  
- Removing unused `release_date` field from seed data
- Updating all API types, models, and tests to match new structure
- Maintaining backwards compatibility in API responses

This change makes the schema more intuitive and reduces nesting depth.

docs/reference/cli/commands.md

@@ -45,7 +45,7 @@ mcp-publisher init [options]
 {
   "name": "io.github.username/server-name",
   "description": "TODO: Add server description",
-  "version_detail": { "version": "1.0.0" },
+  "version": "1.0.0",
   "packages": [
     {
       "registry_type": "npm",