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

tools/publisher/server.json

@@ -1,4 +1,5 @@
 {
+    "$schema": "https://static.modelcontextprotocol.io/schemas/2025-07-09/server.schema.json",
     "description": "<your description here>",
     "name": "io.github.<owner>/<server-name>",
     "status": "active",