This PR adds comprehensive Copilot instructions to help GitHub Copilot better understand and work with the ptcpdump codebase.

What's Changed

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

• Project Overview: Description of ptcpdump as an eBPF-powered packet analyzer with process/container/pod metadata annotation
• Development Environment: Required tools (Go, Clang/LLVM, Bison, etc.) and setup instructions
• Build & Test Commands: Clear commands for building (static/dynamic linking), testing (unit/e2e), and linting
• Code Structure: Directory layout and purpose of key packages (bpf/, cmd/, internal/)
• Coding Standards: Guidelines for Go formatting, C/eBPF code formatting with clang-format, and test conventions
• eBPF-Specific Considerations: Important notes about kernel-space limitations, bytecode generation workflow, and cross-architecture testing
• Container/Kubernetes Integration: Requirements for Docker, containerd, and Kubernetes metadata extraction
• Performance Guidelines: Best practices for high-throughput packet capture
• Common Development Tasks: Step-by-step workflows for adding features, fixing bugs, and modifying eBPF code
• CI/CD Information: GitHub Actions (x86_64) and CircleCI (arm64) test infrastructure
• Documentation Structure: Guide to README files, Hugo docs site, and CHANGELOG

Why This Matters

Following GitHub's best practices for Copilot coding agents, this instructions file will:

1. Help Copilot provide more accurate code suggestions tailored to ptcpdump's architecture
2. Guide contributors on proper development workflows, especially for eBPF code
3. Ensure consistency with the project's coding standards and testing requirements
4. Serve as a reference for both human developers and AI assistants

Closes the issue requesting Copilot instructions setup.

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