Update prompt file to use 'agent' key format

[rmoreirao]

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Source	Review	Production	What Changed
copilot/tutorials/customization-library/prompt-files/your-first-prompt-file.md	fpt ghec	fpt ghec

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[Copilot]

Pull Request Overview

This PR attempts to update the YAML frontmatter format in a prompt file tutorial from mode: 'agent' to agent: 'agent'. However, this change introduces an inconsistency with all other prompt file examples in the repository.

Key issue:

• This change conflicts with the established pattern used in all other prompt file examples, which consistently use mode: 'agent'

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[Copilot]

This change introduces an inconsistency with other prompt files in the same directory. All other prompt files (create-readme.md, document-api.md, generate-unit-tests.md, onboarding-plan.md, and review-code.md) use mode: 'agent' in their YAML frontmatter, not agent: 'agent'.

Additionally, the value agent: 'agent' appears semantically redundant. The key should be mode with the value 'agent' to indicate the mode of operation.

Please update this to use mode: 'agent' to maintain consistency with the other prompt file examples.

Suggested change

	agent: 'agent'
	mode: 'agent'