Update agent frontmatter options to remove deprecated "infer"

[Anddd7]

• Removed 'infer' option and replaced it with 'user-invocable' and 'disable-model-invocation'.
• Clarified the purpose of new options for better usability.
• Updated guidelines to reflect changes in agent invocation behavior.

Pull Request Checklist

• I have read and followed the CONTRIBUTING.md guidelines.
• I have read and followed the Guidance for submissions involving paid services.
• My contribution adds a new instruction, prompt, agent, skill, or workflow file in the correct directory.
• The file follows the required naming convention.
• The content is clearly structured and follows the example format.
• I have tested my instructions, prompt, agent, skill, or workflow with GitHub Copilot.
• I have run npm start and verified that README.md is up to date.

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Description

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Type of Contribution

• New instruction file.
• New prompt file.
• New agent file.
• New plugin.
• New skill file.
• New agentic workflow.
• Update to existing instruction, prompt, agent, plugin, skill, or workflow.
• Other (please specify):

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Additional Notes

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By submitting this pull request, I confirm that my contribution abides by the Code of Conduct and will be licensed under the MIT License.

[Copilot]

Pull request overview

Updates the custom agent authoring guidelines to remove the deprecated infer frontmatter option and document the newer invocation-related flags, keeping repository guidance aligned with current agent frontmatter capabilities.

Changes:

• Removed infer from the YAML frontmatter example and from the checklist.
• Added documentation for user-invocable and disable-model-invocation options, including default behaviors and usage guidance.