feat(engine): add workflow extension plugin seams

CONCEPTS.md

@@ -174,6 +174,9 @@ A Bundled Plugin must be registered in several independently maintained surfaces
 ### Plugin Entry
 The single loadable file persisted as a plugin's path and dynamically imported by the loader. The contract is strict: a package directory is never a valid entry (ESM cannot import directories), so every install surface must resolve a concrete file before persisting, preferring the shipped bundle, then a prebuilt output, then raw workspace source. Legacy registrations that stored a directory are healed in place — re-pointed at a resolved entry — the next time the plugin is enabled or auto-installed.
 
+### Workflow Extension
+A plugin-contributed workflow capability registered through the engine rather than hardcoded into core workflow logic: column metadata, movement policies, column work engines, workflow node handlers, task verdict providers, or merge-routing facts. A Workflow Extension is opt-in by workflow metadata and must degrade or park by an explicit fallback policy when its plugin is disabled or missing, preserving the Default workflow baseline when no extension is active.
+
 ## Workflow columns & traits
 
 *Behind the `experimentalFeatures.workflowColumns` flag. With the flag off, the legacy fixed pipeline (the closed column enum + `VALID_TRANSITIONS`) is authoritative and unchanged.*