feat(skill-execution): implement execute_skill tool with full execution chain

- Add ExecuteSkillTool for LLM to call skills during conversation
- Implement SkillExecutor trait in Kernel for skill execution
- Update AgentLoop to support tool execution with skill_executor
- Add default skills_dir configuration in KernelConfig
- Connect frontend skillMarketStore to backend skill_list command
- Update technical documentation with Skill system architecture

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/zclaw-runtime/src/tool/builtin.rs

@@ -4,13 +4,15 @@ mod file_read;
 mod file_write;
 mod shell_exec;
 mod web_fetch;
+mod execute_skill;
 
 pub use file_read::FileReadTool;
 pub use file_write::FileWriteTool;
 pub use shell_exec::ShellExecTool;
 pub use web_fetch::WebFetchTool;
+pub use execute_skill::ExecuteSkillTool;
 
-use crate::tool::{ToolRegistry, Tool};
+use crate::tool::ToolRegistry;
 
 /// Register all built-in tools
 pub fn register_builtin_tools(registry: &mut ToolRegistry) {
@@ -18,4 +20,5 @@ pub fn register_builtin_tools(registry: &mut ToolRegistry) {
     registry.register(Box::new(FileWriteTool::new()));
     registry.register(Box::new(ShellExecTool::new()));
     registry.register(Box::new(WebFetchTool::new()));
+    registry.register(Box::new(ExecuteSkillTool::new()));
 }