[bentsherman]

Closes #192

The language server can now check unpublished plugins. It reads the spec of an installed plugin from <plugins dir>/<name>-<version>/classes/META-INF/spec.json, the same JSON the registry returns in its spec field.

• The plugins dir is resolved like Nextflow: $NXF_PLUGINS_DIR, then $NXF_HOME/plugins, then ~/.nextflow/plugins.
• A plugin with a version uses the installed release if there is one. Otherwise it is fetched from the registry.
• A plugin without a version is fetched from the registry. If the registry doesn't have it or can't be reached, the latest installed release is used.
• A blank or invalid local spec is treated as empty, so the plugin's includes are still reported instead of staying pending.

LocalPluginRegistryClient wraps HttpPluginRegistryClient, so PluginSpecs (caching, retries) is unchanged. Spec parsing moved into PluginSpec.fromJson so both clients share it.

Limitation: specs are cached for the session, so reinstalling the same version needs a restart of the language server.

Testing

• Unit tests in LocalPluginRegistryClientTest.
• Manual test with nf-prov built from a checkout and installed under an unpublished version (1.9.0): config options are checked and hover shows their descriptions.
• Manual test with nf-schema installed under an unpublished version (2.99.0): plugin includes resolve and calls are checked. The diagnostics match the published 2.6.1 from the registry.