docs: add the copilot instruction

[zhiying-lin]

What type of PR is this?
add the copilot instruction to make our agent even more smart

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Requirements:

• run make reviewable for basic local test
• Read and followed fleet-networking's Code of conduct.
• uses conventional commit messages
• includes documentation
• adds unit tests

How has this code been tested

Special notes for your reviewer

[Copilot]

Pull Request Overview

Adds a new Copilot instruction document to guide AI interactions, including repository context, coding and testing rules, domain knowledge, specifications, and a breadcrumb protocol.

• Introduces general and testing guidelines specific to this Go/Kubernetes monorepo
• Defines terminology, directory structure, and domain/specification file usage
• Establishes a collaborative breadcrumb protocol for planning and documentation

Comments suppressed due to low confidence (2)

.github/copilot-instructions.md:8

• This instruction is contradictory: asking to wait for confirmation but then proceeding without prompting. Consider clarifying the desired behavior.

- If you're waiting for my confirmation ("OK"), proceed without further prompting.

.github/copilot-instructions.md:28

• [nitpick] Nested list indentation is inconsistent. Use a uniform indent (e.g., two or four spaces) for all sub-items to improve readability.

  - `charts/hub-net-controller-manager/` - Helm chart for the fleet hub networking components.