fix(arborist): drop self-link materialization for undeclared workspaces

[manzoorwanijk]

In continuation of our exploration of using install-strategy=linked in the Gutenberg monorepo, which powers the WordPress Block Editor.

Summary

In install-strategy=linked, every workspace not listed in the root package.json's deps was getting a self-symlink inside its own node_modules/ (e.g. packages/test/node_modules/@namespace/test -> ..).
The workspace did not declare itself as a dependency; the link was a side effect of #9076 parking the workspace IsolatedLink off-root.
pnpm does not create such a self-link — packages that need to self-resolve by name are expected to use Node's exports-based self-referencing.

Fix

Keep the workspace IsolatedLink in the tree (so --workspace filters resolve undeclared workspaces and install scripts still run via the diff's unchanged-link path), but mark it isUndeclaredWorkspaceLink = true and treat it as tree-only:

• #extractOrLink returns early for these links, so no symlink is materialized on disk.
• #cleanOrphanedStoreEntries excludes them from the valid set, so any stale self-link from an older npm version is swept on the next install.
• The link is no longer added to workspace.children.

The only behavior change is that require('<self-name>') from inside an undeclared workspace without an exports field no longer resolves.
This matches pnpm and the strict-isolation intent of #9076.
Cross-workspace dep links, declared-workspace root symlinks, and workspace postinstall execution are unaffected.

References

Fixes #9398
Related to #9076