Add optional title field to server.json schema (#608)

## Summary

Introduces a new optional `title` field for MCP servers to provide a
clean, human-readable display name that clients and subregistries can
use for display purposes.

Fixes #606

## Changes

- Added `title` field to `ServerJSON` type (optional, max 100 chars)
- Added validation to reject titles ending with 'MCP Server', 'MCP
server', or 'MCP' suffixes
- Updated JSON schema and OpenAPI specification
- Added comprehensive test coverage for title validation
- Updated all documentation examples to demonstrate the new field

## Validation

The title field:
- Is optional (can be omitted)
- Must not be only whitespace if provided
- Must not end with MCP-related suffixes (case-insensitive)
- Can contain 'MCP' in the middle (e.g., "MCP Weather API" is valid)

Examples:
- ✅ Valid: "GitHub", "Weather API", "Slack Integration"
- ❌ Invalid: "GitHub MCP", "Weather MCP Server", "Slack-MCP"

---------

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

internal/validators/validators.go

@@ -83,6 +83,11 @@ func ValidateServerJSON(serverJSON *apiv0.ServerJSON) error {
 		return err
 	}
 
+	// Validate title if provided
+	if err := validateTitle(serverJSON.Title); err != nil {
+		return err
+	}
+
 	// Validate all packages (basic field validation)
 	// Detailed package validation (including registry checks) is done during publish
 	for _, pkg := range serverJSON.Packages {
@@ -156,6 +161,20 @@ func validateWebsiteURL(websiteURL string) error {
 	return nil
 }
 
+func validateTitle(title string) error {
+	// Skip validation if title is not provided (optional field)
+	if title == "" {
+		return nil
+	}
+
+	// Check that title is not only whitespace
+	if strings.TrimSpace(title) == "" {
+		return fmt.Errorf("title cannot be only whitespace")
+	}
+
+	return nil
+}
+
 func validatePackageField(obj *model.Package) error {
 	if !HasNoSpaces(obj.Identifier) {
 		return ErrPackageNameHasSpaces