✨ Add GitHub Copilot instructions for Kuma repository

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help AI coding assistants provide better context-aware assistance when working on the Kuma service mesh codebase.

What's Added

Created .copilot-instructions.md with detailed guidance covering:

• Project Overview: Description of Kuma as a CNCF service mesh built with Go and Envoy
• Architecture Components: Control plane (kuma-cp), data plane (kuma-dp), CLI (kumactl), and multi-zone capabilities
• Development Workflow: Essential commands for development including:
  • make check - code formatting, linting, and validation
  • make test - running tests with component-specific variants
  • make generate - code generation for protobuf, resources, and policies
  • make test UPDATE_GOLDEN_FILES=1 - updating test golden files
• Testing Framework: Ginkgo BDD testing with golden file patterns
• Code Organization: Clear mapping of pkg/ directory structure and component responsibilities
• Best Practices: Guidelines for code style, testing, performance, and security considerations
• Common Issues & Solutions: Build problems, test failures, and multi-zone complexity handling

Benefits

This will enable GitHub Copilot and other AI coding assistants to:

• Understand Kuma's complex service mesh architecture
• Suggest appropriate make commands for different development tasks
• Provide context-aware code suggestions that respect Kuma's patterns
• Help with testing workflows including golden file management
• Guide developers through the multi-zone and universal deployment complexities

All commands and patterns referenced in the instructions have been verified against the actual repository structure and Makefile targets.

Fixes #14535.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[github-actions]

Reviewer Checklist

🔍 Each of these sections need to be checked by the reviewer of the PR 🔍:
If something doesn't apply please check the box and add a justification if the reason is non obvious.

• Is the PR title satisfactory? Is this part of a larger feature and should be grouped using > Changelog?
• PR description is clear and complete. It Links to relevant issue as well as docs and UI issues
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as an image registry)
• IPv6 is taken into account (.e.g: no string concatenation of host port)
• Tests (Unit test, E2E tests, manual test on universal and k8s)
  • Don't forget ci/ labels to run additional/fewer tests
• Does this contain a change that needs to be notified to users? In this case, UPGRADE.md should be updated.
• Does it need to be backported according to the backporting policy? (this GH action will add "backport" label based on these file globs, if you want to prevent it from adding the "backport" label use no-backport-autolabel label)

[bartsmykla]

Few nits, but lgtm overall

[bartsmykla]

Nit:

Suggested change

	### Multi-zone/Multi-mesh Complexity
	- **Zone configuration**: Test cross-zone functionality carefully
	### Multi-zone/Multi-mesh Complexity
	- **Zone configuration**: Test cross-zone functionality carefully

[bartsmykla]

Nit:

Suggested change

	### Build Issues
	- **Missing dependencies**: Run `make install` to install dev tools via mise
	### Build Issues
	- **Missing dependencies**: Run `make install` to install dev tools via mise

[bartsmykla]

Nit:

Suggested change

	### Test Issues
	- **Golden file mismatches**: Run `make test UPDATE_GOLDEN_FILES=1` to update expected output
	### Test Issues
	- **Golden file mismatches**: Run `make test UPDATE_GOLDEN_FILES=1` to update expected output