Improved custom chat mode and Copilot instructions file

[pamelafox]

Purpose

This PR improves the experience of using GitHub Copilot with the repo, both when assigning issues to the GitHub Copilot Coding Agent and when using GitHub Copilot Chat in VS Code.

• Fixer mode: Renamed debug mode to fixer mode, refined instructions and tools based off more experiments.
• copilot-instructions.md: File that will be checked by GitHub Copilot (all varieties) for overall repo-level instructions. Includes overall code layout and most common tasks.

Does this introduce a breaking change?

When developers merge from main and run the server, azd up, or azd deploy, will this produce an error?
If you're not sure, try it out on an old environment.

[ ] Yes
[X] No

Does this require changes to learn.microsoft.com docs?

This repository is referenced by this tutorial
which includes deployment, settings and usage instructions. If text or screenshot need to change in the tutorial,
check the box below and notify the tutorial author. A Microsoft employee can do this for you if you're an external contributor.

[ ] Yes
[X] No

Type of change

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[X] Documentation content changes
[X] Other... Please describe:

Code quality checklist

See CONTRIBUTING.md for more details.

N/A

[Copilot]

Pull Request Overview

This PR improves the GitHub Copilot integration experience for the repository by introducing general repository-level instructions and refining a specialized chat mode for fixing issues.

• Adds comprehensive copilot-instructions.md file providing development patterns and common tasks
• Renames and refines debug mode to fixer mode with more focused instructions
• Removes the previous debug.chatmode.md file in favor of the improved fixer mode

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
.github/copilot-instructions.md	New file providing repo-level instructions for common development tasks like adding data, environment variables, settings, and tests
.github/chatmodes/fixer.chatmode.md	New fixer mode replacing debug mode with refined instructions and updated tool list
.github/chatmodes/debug.chatmode.md	Removed debug mode file in favor of the new fixer mode

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_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}