fix: pin exact yjs, y-prosemirror, and lib0 versions

Force all packages to use exact versions to reduce duplication.
Note: Velt packages may still bundle their own Yjs internally.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: yoen-velt

package.json

@@ -48,8 +48,9 @@
       "@veltdev/codemirror-crdt-react": "4.5.8",
       "@veltdev/reactflow-crdt": "4.5.8",
       "@veltdev/react": "^4.6.10",
-      "yjs": "^13.6.18",
-      "y-prosemirror": "^1.2.12"
+      "yjs": "13.6.18",
+      "y-prosemirror": "1.2.12",
+      "lib0": "0.2.98"
     }
   }
 }