fix: align skill tool TypeScript types with JSON schema

Update the args parameter in NativeToolArgs and SkillParams to allow
null in addition to string, matching the JSON schema definition where
args has type: ['string', 'null'] and is in the required array.

This aligns the TypeScript interface with the OpenAI strict mode
pattern used by other tools like codebase_search and execute_command.

Author: hannesrudolph

src/core/tools/SkillTool.ts

@@ -5,7 +5,7 @@ import type { ToolUse } from "../../shared/tools"
 
 interface SkillParams {
 	skill: string
-	args?: string
+	args?: string | null
 }
 
 export class SkillTool extends BaseTool<"skill"> {

src/shared/tools.ts

@@ -106,7 +106,7 @@ export type NativeToolArgs = {
 	codebase_search: { query: string; path?: string }
 	generate_image: GenerateImageParams
 	run_slash_command: { command: string; args?: string }
-	skill: { skill: string; args?: string }
+	skill: { skill: string; args?: string | null }
 	search_files: { path: string; regex: string; file_pattern?: string | null }
 	switch_mode: { mode_slug: string; reason: string }
 	update_todo_list: { todos: string }