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-runtime/src/tool/builtin.rs

@@ -8,6 +8,7 @@ mod execute_skill;
 mod skill_load;
 mod path_validator;
 mod task;
+mod ask_clarification;
 
 pub use file_read::FileReadTool;
 pub use file_write::FileWriteTool;
@@ -17,6 +18,7 @@ pub use execute_skill::ExecuteSkillTool;
 pub use skill_load::SkillLoadTool;
 pub use path_validator::{PathValidator, PathValidatorConfig};
 pub use task::TaskTool;
+pub use ask_clarification::AskClarificationTool;
 
 use crate::tool::ToolRegistry;
 
@@ -28,4 +30,5 @@ pub fn register_builtin_tools(registry: &mut ToolRegistry) {
     registry.register(Box::new(WebFetchTool::new()));
     registry.register(Box::new(ExecuteSkillTool::new()));
     registry.register(Box::new(SkillLoadTool::new()));
+    registry.register(Box::new(AskClarificationTool::new()));
 }