Fix 'Orgin' -> 'Origin' typo in action.yml description; delete PLAN.md

PLAN.md tracked the April 2026 improvement work; every phase it listed
is now merged and documented in CHANGELOG.md. Deleting to keep the repo
root uncluttered — the history survives in git log.

Author: iainmcgin

PLAN.md

@@ -1,5 +1,5 @@
 name: "CLA assistant lite"
-description: "An action to handle the Contributor License Agreement (CLA) and Developer Certificate of Orgin (DCO)"
+description: "An action to handle the Contributor License Agreement (CLA) and Developer Certificate of Origin (DCO)"
 author: "SAP"
 branding:
   icon: "award"