Fix misleading server.json schema URL label (#803)

The schema in this repo may contain unreleased changes, so labeling it
with a dated version URL is misleading. Change to use a `draft` URL
instead, and add documentation for the schema contribution process.

- Update $id in server.schema.json to use draft URL
- Update example $schema URL in openapi.yaml
- Add Draft (Unreleased) section to CHANGELOG.md
- Add CONTRIBUTING.md with process for making/releasing schema changes

Written by Claude Code, reviewed and tweaked by me.

Co-authored-by: Claude <noreply@anthropic.com>

tools/extract-server-schema/main.go

@@ -88,8 +88,12 @@ func main() {
 		}
 	}
 
-	// Build the JSON Schema document with dynamic version from OpenAPI spec
-	schemaID := fmt.Sprintf("https://static.modelcontextprotocol.io/schemas/%s/server.schema.json", version)
+	// Build the JSON Schema document with draft URL
+	// The in-repo schema uses "draft" since it may contain unreleased changes.
+	// When releasing, the schema is published to a versioned URL (e.g., 2025-10-17)
+	// on https://github.com/modelcontextprotocol/static
+	_ = version // version from OpenAPI spec available if needed
+	schemaID := "https://static.modelcontextprotocol.io/schemas/draft/server.schema.json"
 	jsonSchema := map[string]interface{}{
 		"$comment":    "This file is auto-generated from docs/reference/api/openapi.yaml. Do not edit manually. Run 'make generate-schema' to update.",
 		"$schema":     "http://json-schema.org/draft-07/schema#",