feat: ask_clarification tool + clarification system prompt + progressive skill loading fix

- New ask_clarification tool (crates/zclaw-runtime/src/tool/builtin/ask_clarification.rs)
  with 5 clarification types: missing_info, ambiguous_requirement, approach_choice, risk_confirmation, suggestion
- Registered as built-in tool in builtin.rs
- Added clarification system prompt instructions to messaging.rs system prompt
- Fixed messaging.rs skill injection: when SkillIndexMiddleware is active,
  only inject usage instructions (not full skill list), avoiding duplicate injection
- Fixed pre-existing unicode arrow character causing string literal parse error

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

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

@@ -275,7 +275,7 @@ impl Kernel {
                 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");
+                prompt.push_str("User: 分析腾讯财报 -> Intent: Financial analysis -> Call: execute_skill(\"finance-tracker\", {...})\n");
             }
         }
 
@@ -294,6 +294,19 @@ impl Kernel {
             prompt.push_str("- Maximum 3 concurrent sub-agents — batch if more are needed\n");
         }
 
+        // Clarification system — always enabled
+        prompt.push_str("\n\n## Clarification System\n\n");
+        prompt.push_str("When you encounter any of the following situations, call `ask_clarification` to ask the user BEFORE proceeding:\n\n");
+        prompt.push_str("- **Missing information**: User's request is critical details you you need but don't have\n");
+        prompt.push_str("- **Ambiguous requirement**: Multiple valid interpretations exist\n");
+        prompt.push_str("- **Approach choice**: Several approaches with different trade-offs\n");
+        prompt.push_str("- **Risk confirmation**: Action could have significant consequences\n\n");
+        prompt.push_str("### Guidelines:\n");
+        prompt.push_str("- ALWAYS prefer asking over guessing\n");
+        prompt.push_str("- Provide clear options when possible\n");
+        prompt.push_str("- Include brief context about why you're asking\n");
+        prompt.push_str("- After receiving clarification, proceed immediately\n");
+
         prompt
     }