fix(web): #107 review — skills.js carries // @ts-check, as CLAUDE.md asks of every JS file

The file had no `// @ts-check` header, so an editor or a plain `tsc --allowJs` never type-checks it.
It needs no other change: `tsc --noEmit --allowJs --checkJs --skipLibCheck --target es2022 --module
commonjs --types node web/ai-extension/skills.js` (TypeScript 6.0 from the Code-OSS checkout, whose
defaults are strict) reports nothing, before and after, and `tsc --allowJs` without --checkJs now
checks it too and reports nothing.

No unit test: the header changes no behaviour (web/test/unit/skills.test.js passes as before); the
check is the tsc run above. The other files in web/ai-extension have no header either; they are left
as they are.

Author: ndemianc

web/ai-extension/skills.js

@@ -7,6 +7,7 @@
 // the desktop's files are untouched (this is applied to the embedded copy, like copy.js and skin.js).
 //
 // Plain CommonJS; web/test/unit/skills.test.js pins it.
+// @ts-check
 'use strict';
 
 const PREAMBLE = [