discoverFromUserBundlesDir

Discover bundles the user has installed into ~/.ksl/bundles/ via BundleLoader.loadDirectory and append them to loadedBundles. The directory is created if it does not yet exist, giving users a well-known place to drop bundle JARs (e.g. the KSL Book Examples bundle). Typically called once from the host's init {} block in place of discoverFromClasspath, so a released app ships with no baked-in bundles yet still loads whatever the user installed.

No-op when the directory holds no loadable KSLModelBundle JARs. When at least one is found, onBundlesChanged fires after the append.