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-kernel/src/config.rs

@@ -5,11 +5,12 @@
 //! - No provider prefix or alias mapping
 //! - Simple, unified configuration structure
 
+use std::path::PathBuf;
 use std::sync::Arc;
 use serde::{Deserialize, Serialize};
 use secrecy::SecretString;
-use zclaw_types::{Result, ZclawError};
-use zclaw_runtime::{LlmDriver, AnthropicDriver, OpenAiDriver, GeminiDriver, LocalDriver};
+use zclaw_types::Result;
+use zclaw_runtime::{LlmDriver, AnthropicDriver, OpenAiDriver};
 
 /// API protocol type
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
@@ -119,6 +120,10 @@ pub struct KernelConfig {
     /// LLM configuration
     #[serde(flatten)]
     pub llm: LlmConfig,
+
+    /// Skills directory path (optional, defaults to ./skills)
+    #[serde(default)]
+    pub skills_dir: Option<PathBuf>,
 }
 
 fn default_database_url() -> String {
@@ -147,10 +152,18 @@ impl Default for KernelConfig {
                 max_tokens: default_max_tokens(),
                 temperature: default_temperature(),
             },
+            skills_dir: default_skills_dir(),
         }
     }
 }
 
+/// Default skills directory (./skills relative to cwd)
+fn default_skills_dir() -> Option<std::path::PathBuf> {
+    std::env::current_dir()
+        .ok()
+        .map(|cwd| cwd.join("skills"))
+}
+
 impl KernelConfig {
     /// Load configuration from file
     pub async fn load() -> Result<Self> {
@@ -321,6 +334,7 @@ impl KernelConfig {
         Self {
             database_url: default_database_url(),
             llm,
+            skills_dir: None,
         }
     }
 }