fix(deps-graph): keep the existing lockfileVersion on graph merge

Copilot review: convertGraphToLockfile hardcodes lockfileVersion '9.0', so
preferring graph.lockfileVersion in the merge helper could silently downgrade
workspaces whose pnpm writes a newer schema and trigger a full rewrite on the
next install — the opposite of the goal here. Flip the fallback so existing
wins whenever present; graph's value is only used when starting from no
lockfile at all.

Author: zkochan

scopes/dependencies/pnpm/pnpm.package-manager.ts

@@ -546,7 +546,11 @@ function mergeGraphLockfileIntoExisting(existing: LockfileFile, graph: LockfileF
   ).sort();
   const merged = {
     ...existing,
-    lockfileVersion: graph.lockfileVersion ?? existing.lockfileVersion,
+    // Keep the existing lockfile's schema version. convertGraphToLockfile hardcodes
+    // lockfileVersion: '9.0', so preferring graph.lockfileVersion would silently
+    // downgrade workspaces whose pnpm already writes a newer schema and trigger a
+    // full rewrite on the next install.
+    lockfileVersion: existing.lockfileVersion ?? graph.lockfileVersion,
     importers,
     packages: mergeEntryRecords(existing.packages, graph.packages),
     snapshots: mergeEntryRecords(existing.snapshots, graph.snapshots),