Add comprehensive GitHub Copilot instructions for WxJava SDK development

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot with detailed guidance on how to work effectively with the WxJava codebase. The instructions cover all essential aspects of development workflow for this WeChat Java SDK project.

Key Features

Build and Validation Commands - All commands have been thoroughly tested and validated:

• Basic compilation: mvn clean compile -DskipTests=true --no-transfer-progress (4-5 min initial, 50s incremental)
• Full packaging: mvn clean package -DskipTests=true --no-transfer-progress (2-3 min initial, 1 min incremental)
• Code quality validation: mvn checkstyle:check --no-transfer-progress (~2 seconds)
• Dependency analysis and single module builds

Critical Timing Information with "NEVER CANCEL" warnings:

• Measured actual build times and provided appropriate timeout recommendations
• Clear distinction between first-time builds (longer due to dependency downloads) and incremental builds
• Specific timeout values to prevent premature command cancellation

Project Structure Guidance:

• Comprehensive overview of 34 Maven modules including core SDK modules (weixin-java-{common,mp,pay,miniapp,cp,open,channel,qidian})
• Framework integration modules (Spring Boot starters, Solon plugins)
• Navigation paths to key development areas and configuration files

Development Workflow:

• Code style requirements (2 spaces indentation, Google Java Style Guide, checkstyle validation)
• Contributing guidelines (develop branch targeting, PR requirements)
• Test structure explanation (TestNG framework, external WeChat API credentials required, tests disabled by default)
• Quality gates and validation steps required before submitting changes

SDK Usage Patterns:

• Maven dependency examples for different modules
• Common development areas and file organization patterns
• Clear explanation that this is a library/SDK project, not a runnable application
• References to demo projects and integration examples

Validation

Every command and workflow described in the instructions has been tested on a fresh repository clone to ensure accuracy. The instructions follow the imperative tone requirement ("Run this command", "Do not do this") and provide complete command sequences that work reliably.

The file starts with a firm directive to always follow the instructions first before falling back to additional search and context gathering, as requested in the issue requirements.

Fixes #3672.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.