feat: simplify subagent model configuration with model selector

Refactor subagent model configuration from nested modelConfig object to a simple model string field for better UX and clarity.

Changes:
- Replace modelConfig object with model string in SubagentConfig interface
- Add model-selection.ts utility for parsing and validating model selectors
- Support 'inherit' keyword and bare model IDs (e.g., 'glm-5', 'claude-sonnet-4-6')
- Maintain backward compatibility by parsing legacy modelConfig frontmatter
- Update validation to reject cross-provider authType-prefixed selectors
- Update SDK types (TypeScript and Java) to reflect new schema
- Add comprehensive tests for model selection and validation
- Update documentation with model selection examples

Breaking changes:
- modelConfig.frontmatter field deprecated in favor of model field
- Cross-provider model selectors (e.g., 'openai:gpt-4') not supported for subagents

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/sdk-java/qwencode/src/main/java/com/alibaba/qwen/code/cli/protocol/protocol.ts

@@ -534,12 +534,6 @@ export function isToolResultBlock(block: any): block is ToolResultBlock {
 
 export type SubagentLevel = 'session';
 
-export interface ModelConfig {
-  model?: string;
-  temp?: number;
-  top_p?: number;
-}
-
 export interface RunConfig {
   max_time_minutes?: number;
   max_turns?: number;
@@ -552,7 +546,7 @@ export interface SubagentConfig {
   systemPrompt: string;
   level: SubagentLevel;
   filePath?: string;
-  modelConfig?: Partial<ModelConfig>;
+  model?: string;
   runConfig?: Partial<RunConfig>;
   color?: string;
   readonly isBuiltin?: boolean;