fix: address CodeRabbit review — deduplicate prompt, fix skill count

- Swap loadHistory/appendTurn order to prevent user message appearing
  twice in the prompt (once in history, once as USER MESSAGE)
- Calculate actual loaded skill count via fs.existsSync instead of
  counting requested skill names (banner now reflects reality)
- Add err.stack to test harness error output for better debugging

tests/scripts/claw.test.js

@@ -33,6 +33,7 @@ function test(name, fn) {
   } catch (err) {
     console.log(`  \u2717 ${name}`);
     console.log(`    Error: ${err.message}`);
+    if (err.stack) { console.log(`    Stack: ${err.stack}`); }
     return false;
   }
 }