CCA: modernize-dotnet.agent.md should have an auto-update mechanism

### Description

Direct customer feedback:

> Customer manually copies modernize-dotnet.agent.md into their org's .github-private repo. IDE extensions have built-in update mechanisms, but the Copilot Coding Agent (CCA) does not. When the agent file is updated by the modernize-dotnet team, customers have no way to know or pull updates automatically.

> "It would be helpful to keep the modernize-dotnet Agent file updated by GitHub as part of the process. Currently, it is drawing from the file I copy pasted into our org's .github-private repo, but that is a manual process. The IDE extensions have an update mechanism, but CCA does not yet."

Expected Behavior:
The CCA flow should support auto-updating the agent file, or at minimum notify orgs when a newer version is available.