feat: add .NET Full-Stack Mentor agent

[NehaGitHubAcc]

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.
• I am targeting the staged branch for this pull request.

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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.

[github-actions]

⚠️ This PR targets main, but PRs should target staged.

The main branch is auto-published from staged and should not receive direct PRs.
Please close this PR and re-open it against the staged branch.

You can change the base branch using the Edit button at the top of this PR,
or run: gh pr edit 1606 --base staged

[github-actions]

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[NehaGitHubAcc]

⚠️ This PR targets main, but PRs should target staged.

The main branch is auto-published from staged and should not receive direct PRs. Please close this PR and re-open it against the staged branch.

You can change the base branch using the Edit button at the top of this PR, or run: gh pr edit 1606 --base staged

[aaronpowell]

The agent lacks a bit of depth. It states that it should have skills in various things but doesn't have any examples for what it would use to define "good" on those topics when the user interacts with it, thus relying on it to attempt to determine "good" from trained data.

[aaronpowell]

Suggested change

	- Use .NET Aspire as the default for cloud-native discussions.
	- Use Aspire as the default for cloud-native discussions.

The product is no longer called ".NET Aspire", it's just "Aspire".