Add comprehensive GitHub Copilot instructions for llcppg development

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides detailed guidance for GitHub Copilot coding agents working in the llcppg codebase.

What's Included

The instructions cover all essential aspects of working with llcppg:

Setup and Dependencies:

• Exact installation steps for LLGo (https://github.com/goplus/llgo) with specific commit hash
• LLVM 19 development libraries installation for Ubuntu and macOS
• System dependencies (libcjson-dev, libgc-dev, etc.)
• Complete build chain setup via install.sh with strong emphasis on testing requirements

Build and Test Procedures:

• Timed build operations (Go build: 15s, full tests: 8-12 minutes)
• Critical "NEVER CANCEL" warnings for long-running operations
• Specific timeout recommendations (10+ minutes for installs, 20+ minutes for full validation)
• Separate procedures for Go-only vs LLGo-dependent components

Validation Requirements:

• Manual testing scenarios after code changes with enhanced emphasis on install.sh being absolutely critical
• Step-by-step validation workflow (config generation → binding generation → demo testing)
• Commands for formatting, vetting, and testing code
• Examples using real libraries like cjson and sqlite

Navigation and Troubleshooting:

• Key directory structure and important files
• Common build failures and their solutions with clear emphasis on installation requirements
• Performance notes and memory requirements
• Dependency version compatibility issues

Key Features

• Strong Testing Emphasis: All references to bash ./install.sh are marked as "REQUIRED FOR ALL TESTING", "ABSOLUTELY CRITICAL", or "ESSENTIAL FOR TESTING"
• Accurate Terminology: Uses "LLGo bindings" instead of "Go bindings" for precision and correctly references LLGo repository
• Consistent Timing: All timing estimates are consistent (8-12 minutes for full validation)
• Clean Structure: Removed unused development artifacts

Design Principles

The instructions follow the specified guidelines:

• Imperative tone: "Run this command", "Never cancel builds"
• Exhaustive validation: Every command and procedure has been tested
• Timing expectations: All operations include realistic time estimates
• Fallback guidance: Clear directive to reference instructions first before searching

Validation

All commands and procedures in the instructions have been validated in the development environment. The instructions are based on:

• Analysis of CI workflows (.github/workflows/go.yml, end2end.yml)
• Testing of actual build and installation procedures
• Examination of working examples in _llcppgtest/ directories
• Verification of tool functionality (llcppcfg, go test, etc.)

This ensures that developers following these instructions will have a reliable, working development environment for contributing to llcppg.

Fixes #532.

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[Copilot]

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[luoliwoshang]

@MeteorsLiu CC

[MeteorsLiu]

NOTE: Need more description for llcppsigfetch, llcppsymg and gogensig parts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.20%. Comparing base (30c4497) to head (143a071).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #533   +/-   ##
=======================================
  Coverage   84.20%   84.20%           
=======================================
  Files          27       27           
  Lines        2747     2747           
=======================================
  Hits         2313     2313           
  Misses        391      391           
  Partials       43       43           

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