refactor(skills): progressive skill loading — avoid duplicate injection

When SkillIndexMiddleware is active, build_system_prompt_with_skills no
longer injects the full categorized skill list. Instead it only adds
usage instructions, while the middleware handles the lightweight index.
This reduces ~2000 tokens per request for the 75-skill system.

crates/zclaw-kernel/src/kernel/messaging.rs

@@ -229,35 +229,54 @@ impl Kernel {
             .map(|p| p.clone())
             .unwrap_or_else(|| "You are a helpful AI assistant.".to_string());
 
-        // Inject skill metadata only (progressive loading pattern from DeerFlow).
-        // Full skill content is loaded on-demand via `load_skill_content` tool.
+        // Progressive skill loading (DeerFlow pattern):
+        // If the SkillIndexMiddleware is registered in the middleware chain,
+        // it will inject a lightweight index at priority 200.
+        // We still inject a basic instruction block here for when middleware is not active.
+        //
+        // When middleware IS active, avoid duplicate injection by only keeping
+        // the skill-use instructions (not the full list).
+        let skill_index_active = {
+            use zclaw_runtime::tool::SkillExecutor;
+            !self.skill_executor.list_skill_index().is_empty()
+        };
+
         if !skills.is_empty() {
-            prompt.push_str("\n\n## Available Skills\n\n");
-            prompt.push_str("You have access to specialized skills. Analyze user intent and autonomously call `execute_skill` with the appropriate skill_id.\n\n");
+            if skill_index_active {
+                // Middleware will inject the index — only add usage instructions
+                prompt.push_str("\n\n## Skills\n\n");
+                prompt.push_str("You have access to specialized skills listed in the skill index above. ");
+                prompt.push_str("Analyze user intent and autonomously call `skill_load` to inspect a skill, ");
+                prompt.push_str("then `execute_skill` with the appropriate skill_id.\n\n");
+                prompt.push_str("- **IMPORTANT**: Autonomously decide when to use skills based on user intent.\n");
+                prompt.push_str("- Do not wait for explicit skill names — recognize the need and act.\n");
+                prompt.push_str("- If unsure about a skill, call `skill_load` first to understand its parameters.\n");
+            } else {
+                // No middleware — inject full skill list as fallback
+                prompt.push_str("\n\n## Available Skills\n\n");
+                prompt.push_str("You have access to specialized skills. Analyze user intent and autonomously call `execute_skill` with the appropriate skill_id.\n\n");
 
-            // Group skills by category based on their ID patterns
-            let categories = self.categorize_skills(&skills);
-
-            for (category, category_skills) in categories {
-                prompt.push_str(&format!("### {}\n", category));
-                for skill in category_skills {
-                    prompt.push_str(&format!(
-                        "- **{}**: {}",
-                        skill.id.as_str(),
-                        skill.description
-                    ));
+                let categories = self.categorize_skills(&skills);
+                for (category, category_skills) in categories {
+                    prompt.push_str(&format!("### {}\n", category));
+                    for skill in category_skills {
+                        prompt.push_str(&format!(
+                            "- **{}**: {}",
+                            skill.id.as_str(),
+                            skill.description
+                        ));
+                        prompt.push('\n');
+                    }
                     prompt.push('\n');
                 }
-                prompt.push('\n');
-            }
 
-            prompt.push_str("### When to use skills:\n");
-            prompt.push_str("- **IMPORTANT**: You should autonomously decide when to use skills based on your understanding of the user's intent.\n");
-            prompt.push_str("- Do not wait for explicit skill names - recognize the need and act.\n");
-            prompt.push_str("- Match user's request to the most appropriate skill's domain.\n");
-            prompt.push_str("- If multiple skills could apply, choose the most specialized one.\n\n");
-            prompt.push_str("### Example:\n");
-            prompt.push_str("User: \"分析腾讯财报\" → Intent: Financial analysis → Call: execute_skill(\"finance-tracker\", {...})\n");
+                prompt.push_str("### When to use skills:\n");
+                prompt.push_str("- **IMPORTANT**: You should autonomously decide when to use skills based on your understanding of the user's intent.\n");
+                prompt.push_str("- Do not wait for explicit skill names - recognize the need and act.\n");
+                prompt.push_str("- Match user's request to the most appropriate skill's domain.\n\n");
+                prompt.push_str("### Example:\n");
+                prompt.push_str("User: \"分析腾讯财报\" → Intent: Financial analysis → Call: execute_skill(\"finance-tracker\", {...})\n");
+            }
         }
 
         // Sub-agent delegation instructions (Ultra mode only)