Update accessibility information for Copilot Studio agents

Clarified accessibility of agents built in Copilot Studio.

Author: brown9804

3_PowerPlatform/2_CopilotStudio/0_DesignCustomCopilots.md

@@ -10,6 +10,8 @@ Last updated: 2025-07-17
 
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+> Agents you build in Copilot Studio stay within your own Microsoft tenant/subscription by default. They are not publicly accessible unless you explicitly configure them to be shared or published (for example, deploying them to Teams, a website, or another channel).
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 <summary><b>List of References</b> (Click to expand)</summary>