fix(capcut): 优化触发目录扫描时的代码组织

在 `capcut-tracks.js` 中将 `execFile` 的导入移到模块顶部，并修复临时目录清理时的 fs 引用问题

.claude/skills/video-from-script/scripts/lib/capcut-tracks.js

@@ -415,10 +415,10 @@ async function addSlotsLocally(draftUrl, items, timeline, trackId, options = {})
       })
       if (matVideo) {
         items[i]._segmentId = matVideo.id
-        slots.push(buildSlot(matVideo.id, videoTrack.id, i, tl, US))
+        slots.push(buildSlot(matVideo.id, videoTrack.id, i, tl))
       }
     } else {
-      slots.push(buildSlot(segId, videoTrack.id, i, tl, US))
+      slots.push(buildSlot(segId, videoTrack.id, i, tl))
     }
   }
 
@@ -458,12 +458,13 @@ function generateUUID() {
   })
 }
 
+const { execFile } = require('child_process')
+
 function triggerDirScan(dir) {
-  const { execFile } = require('child_process')
   const tmp = dir + '.slot_tmp'
   if (process.platform === 'darwin') {
     execFile('rsync', ['-a', dir + '/', tmp], (err) => {
-      try { require('fs').rmSync(tmp, { recursive: true, force: true }) } catch {}
+      try { fs.rmSync(tmp, { recursive: true, force: true }) } catch {}
     })
   }
 }