Add $schema to server.json specification to enable versioning, IDE autocomplete (#308)

## Summary
- Add optional `$schema` property to ServerDetail specification
- Update JSON Schema, OpenAPI spec, Go models, examples, and publisher
template
- Enable schema versioning and IDE validation support

## Changes
- Added `$schema` as optional string property with URI format in specs
- Updated Go ServerDetail struct with new Schema field
- Added `$schema` to examples and publisher template
- Copy schema file to ../static for GitHub Pages hosting

Fixes #139

internal/model/model.go

@@ -111,6 +111,7 @@ type VersionDetail struct {
 
 // ServerDetail represents complete server information as defined in the MCP spec (pure, no registry metadata)
 type ServerDetail struct {
+	Schema        string        `json:"$schema,omitempty" bson:"$schema,omitempty"`
 	Name          string        `json:"name" bson:"name"`
 	Description   string        `json:"description" bson:"description"`
 	Status        ServerStatus  `json:"status,omitempty" bson:"status,omitempty"`