feat: add workflow engine contribution contracts

docs/MODULE_ARCHITECTURE.md

@@ -148,6 +148,16 @@ Other stable runtime capabilities currently include:
 - `calendar.outbox` and `calendar.scheduling`
 - `poll.scheduling`
 - `notifications.dispatch`
+- `workflow.definitionContributions` and `workflow.runtimeWorker`
+
+Modules contribute reusable process baselines through
+`ModuleManifest.workflow_definitions`. Each contribution pins its origin module
+and version, stable key, schema and content hash, native graph/BPMN content,
+governance ceilings, execution mode, and required capabilities/interfaces.
+`govoplan-workflow-engine` reconciles these declarations idempotently. A module
+upgrade appends a baseline revision without replacing the active revision or
+mutating a local override; the optional `govoplan-workflow` package supplies
+the comparison, derivation, and reset UI.
 
 ### Named Interface Contracts
 
@@ -889,7 +899,7 @@ First slice:
 - `govoplan-files` owns file-backed governed locations and uploaded/stored file
   evidence.
 - `govoplan-reporting` owns report/data views and scheduled outputs.
-- `govoplan-workflow` owns process state, approvals, scheduling of process
+- `govoplan-workflow-engine` owns process state, approvals, scheduling of process
   steps, and human review.
 
 Future `govoplan-datasources` is justified when GovOPlaN needs a broad source