docs(admin): experimental banner + MIT LICENSE (QoL sweep) (#22)

Part of 2026-05-19 multi-repo OSS-readiness QoL sweep.
See: GoCodeAlone/workflow#714

Co-authored-by: Claude Haiku 4.5 <noreply@anthropic.com>

Author: intel352

README.md

@@ -1,5 +1,7 @@
 # workflow-plugin-admin
 
+> ⚠️ **Experimental** — This plugin compiles and passes its unit tests but has not been validated in any active GoCodeAlone-internal production deployment. Use with caution. Please [open an issue](https://github.com/GoCodeAlone/workflow-plugin-admin/issues/new) if you adopt it so we can promote it to **verified** status.
+
 External plugin for the [workflow engine](https://github.com/GoCodeAlone/workflow) that provides the admin dashboard UI and config-driven admin routes.
 
 ## What it does