Add comprehensive GitHub Copilot instructions for kerb4j development

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file to guide GitHub Copilot coding agents in working effectively with the kerb4j codebase.

What's included

The instructions provide exhaustive guidance for developers working with this Kerberos/SPNEGO authentication library:

Validated Build Commands

All commands have been tested and timing measured:

• mvn clean compile -q (9 seconds) - compilation only
• mvn clean package -q (48 seconds) - full build with tests
• mvn test -q (24 seconds) - tests only
• mvn clean install -DskipTests -q (28 seconds) - install without tests

Critical Timing and Safety Guidelines

• NEVER CANCEL warnings with explicit 120+ minute timeout recommendations
• Detailed timing expectations to prevent premature build cancellation
• Emphasis on waiting for test completion as they involve real Kerberos server startup

Project Structure Documentation

Complete overview of all 11 modules:

• kerb4j-base64/ - Base64 encoding utilities
• kerb4j-common/ - Core Kerberos functionality and test infrastructure
• kerb4j-client/ - Client-side SPNEGO/Kerberos authentication
• kerb4j-server/ - Server-side authentication (Spring Security, Tomcat, LDAP)

Validation Scenarios

Specific guidance for testing changes:

• KDC server startup and authentication flow validation
• Spring Boot integration testing with embedded Tomcat
• SPNEGO negotiation and HTTP authentication workflows
• Group extraction from Microsoft AD PAC certificates

Key Classes and Development Patterns

Documentation of important classes like SpnegoClient, SpnegoContext, KerberosSecurityTestcase, and proper usage patterns for thread-safe ticket caching.

Validation Process

Every command and instruction was thoroughly validated:

• ✅ All build commands execute successfully
• ✅ Tests demonstrate real Kerberos authentication with "Successfully validated" messages
• ✅ Integration tests start embedded servers and complete full workflows
• ✅ Timing measurements reflect actual execution on the target environment

The instructions follow the imperative tone requirement and prioritize working, validated commands over theoretical guidance.

Fixes #69.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• server.springsource.org
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java -jar /home/REDACTED/work/kerb4j/kerb4j/kerb4j-server/kerb4j-server-spring-security/target/surefire/surefirebooter-20250830080023845_24.jar /home/REDACTED/work/kerb4j/kerb4j/kerb4j-server/kerb4j-server-spring-security/target/surefire 2025-08-30T07-59-46_323-jvmRun1 surefire-20250830080023845_22tmp surefire_4-20250830080023845_23tmp (dns block)
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java -jar /home/REDACTED/work/kerb4j/kerb4j/kerb4j-server/kerb4j-server-spring-security/target/surefire/surefirebooter-20250830080059520_24.jar /home/REDACTED/work/kerb4j/kerb4j/kerb4j-server/kerb4j-server-spring-security/target/surefire 2025-08-30T08-00-46_614-jvmRun1 surefire-20250830080059520_22tmp surefire_4-20250830080059520_23tmp (dns block)
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java -jar /home/REDACTED/work/kerb4j/kerb4j/kerb4j-server/kerb4j-server-spring-security/target/surefire/surefirebooter-20250830080525756_24.jar /home/REDACTED/work/kerb4j/kerb4j/kerb4j-server/kerb4j-server-spring-security/target/surefire 2025-08-30T08-04-53_814-jvmRun1 surefire-20250830080525756_22tmp surefire_4-20250830080525756_23tmp (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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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.