fix: skip managed workspace module links when bundled packages are absent #358

The #347 workspace-command path now calls ensureNodeModulesSymlink using NODE_PATH from runtime metadata. The documented SKIP_JS_PACKAGES=1 build still writes NODE_PATH but skips installation of its node_modules directory. ensureNodeModulesSymlink does not verify the target exists before creating the workspace link, unlike the runtime launch scripts that guard with a directory check.

A fresh attached workspace therefore gets a dangling root node_modules link into the read-only runtime mount and cannot create its own local node_modules or complete npm install normally. Verify the target is an existing directory before creating or replacing a managed link. Preserve real restored project node_modules and leave ordinary user-managed links alone.

Add coverage for SKIP_JS_PACKAGES / nonexistent runtime module directories, while retaining existing restored-module and managed-link migration tests. Validated from build-packages.sh, rememberPreferredNodeModules, ensureNodeModulesSymlink, and executeWorkspaceCommand after a Codex review finding during import.