docs: add Premium license warning for external_agent resource

Co-authored-by: M Atif Ali <U04T3LN8ASU+AtifCoder@users.noreply.github.com>

Author: blink-so[bot]

docs/resources/external_agent.md

@@ -10,7 +10,8 @@ description: |-
 
 Define an external agent to be used in a workspace.
 
-
+> [!WARNING]
+> External agents require a Premium Coder license. This feature is not available with the free tier.
 
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