chore(deno): finish import extensions for sites/mp-sync paths

- sites/mp-sync relative imports already carry extensions; drop the biome
  useImportExtensions override and --unstable-sloppy-imports in typecheck:deno
- mp-sync scripts: deno shebang (+x), static `import YAML from "yaml"`,
  README/usage switched to `deno run -A`
- verified: check.mjs --to e6820a8 (49 unchanged), movieclaw-import --check (0 diff)

Co-Authored-By: Claude Fable 5.1 <[email protected]>

biome.json

@@ -65,16 +65,6 @@
           }
         }
       }
-    },
-    {
-      "includes": ["packages/adapters/src/sites/**", "packages/testkit/src/sites/**", "scripts/mp-sync/**"],
-      "linter": {
-        "rules": {
-          "correctness": {
-            "useImportExtensions": "off"
-          }
-        }
-      }
     }
   ]
 }