docs: clarify compute-core contract boundary (#3)

Author: intel352

README.md

@@ -3,10 +3,16 @@
 Public Workflow plugin and Go module for compute protocol and provider catalog
 contracts.
 
-Provider plugins use this module for shared JSON-compatible protocol types,
-validation helpers, and canonical hashing. They also declare a plugin dependency
-on `workflow-plugin-compute-core` in `plugin.json`, giving Workflow a registry
-dependency anchor separate from runtime execution plugins.
+Provider plugins use this module for shared compute provider-catalog data
+types, validation helpers, and canonical hashing. They also declare a plugin
+dependency on `workflow-plugin-compute-core` in `plugin.json`, giving Workflow a
+registry dependency anchor separate from runtime execution plugins.
+
+This is distinct from the Workflow plugin runtime contract: external plugins
+still expose their runtime capabilities through Workflow's gRPC/protobuf plugin
+service contracts. The provider-catalog structs in `protocol/` are the typed
+declaration data that provider plugins publish and `workflow-plugin-compute`
+validates.
 
 This plugin intentionally advertises no module, step, trigger, or IaC runtime
 capabilities.