Add MIT license reference to README

[Copilot]

Addresses review feedback on #6625 to add an explicit MIT license reference to the README, following the pattern used by other Microsoft OSS projects.

Changes

• Replaced generic license text in README.md with standard MIT license reference linking to the LICENSE file
• Follows the same format as dotnet/aspire and microsoft/vscode repositories

## License

Licensed under the [MIT](LICENSE) license.

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