Add LLM detection telemetry documentation for .NET CLI

[Copilot]

Summary

This pull request updates the .NET CLI telemetry documentation to include details about the new LLM agent detection feature. The most important changes clarify what data is collected when the CLI is run by an LLM agent and provide specifics on how detection works.

Telemetry feature update

• Added documentation that, starting from version >=10.0.100, the CLI reports the detected LLM agent name if invoked from an LLM agent. This includes a reference to the new LLM Detection section for more information.

New LLM Detection section

• Introduced a new "LLM Detection" section explaining how the CLI detects LLM agents and which environment variables are checked. The section includes a table listing supported agents (GitHub Copilot, Claude Code, Cursor, Windsurf, Cline, Roo Code, and Zed) and the environment variables used for detection, clarifying that only the agent type is reported, not the actual variable values.

Fixes #48774

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

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Internal previews

📄 File	🔗 Preview link
docs/core/tools/telemetry.md	.NET SDK and .NET CLI telemetry

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

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Internal previews

📄 File	🔗 Preview link
docs/core/tools/telemetry.md	docs/core/tools/telemetry

[meaghanlewis]

@baronfel let me know if these changes are what you had in mind for the LLM telemetry data point. Should I also include Cursor in the agent list, and a scenario for when multiple agents are used?

[baronfel]

@copilot use the additional detections from dotnet/sdk#51054 (especially the test cases here) to flesh out this table since we added more after this issue was originally logged.

[Copilot]

Updated the table with additional LLM agents including GitHub Copilot, Cursor, Windsurf, Cline, Roo Code, and Zed based on the detections in SDK PR #51054. See commit 8da2d42.

[baronfel]

@meaghanlewis thanks for kicking this off! We added a few more LLM agent detections recently so I'm having Copilot tackle those as well. Other than that this looks great.