fix: stop re-escaping non-ASCII in package.json when enumerating a test

CI step `format` failed (pipeline 2255, exit 1) -- not on the test, on formatting.

I enumerated the new test by loading packages/mosaic/package.json with python's
json module and writing it back. json.dumps defaults to ensure_ascii=True, so the
em-dash in

    "description": "Mosaic agent framework — installation wizard and meta package"

was rewritten as —. Valid JSON, identical when parsed, and rejected by
`pnpm format:check`.

Enumeration is a TEXT edit now: read the file, splice the test into the
test:framework-shell string, write it back. A JSON round-trip rewrites the whole
document to serialiser defaults; the only safe edit to a formatted file is one
that touches the bytes it means to touch.

diff vs main is now exactly one line in each branch.

packages/mosaic/package.json

@@ -6,7 +6,7 @@
     "url": "https://git.mosaicstack.dev/mosaicstack/stack.git",
     "directory": "packages/mosaic"
   },
-  "description": "Mosaic agent framework \u2014 installation wizard and meta package",
+  "description": "Mosaic agent framework — installation wizard and meta package",
   "type": "module",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",