Add comprehensive GitHub Copilot instructions for helm-diff development

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help coding agents work effectively with the helm-diff codebase. The instructions provide detailed guidance on building, testing, and developing the Helm plugin.

Key Features

Complete Development Workflow:

• Bootstrap dependencies with make bootstrap (~50s first time, <1s cached)
• Build with linting via make build (~9s)
• Run comprehensive tests with make test (~12s, 38.7% coverage)
• Code formatting and quality checks

Validated Commands and Timing:
All commands have been tested and measured with appropriate timeout recommendations:

• Never cancel builds - proper timeout values provided for all operations
• PATH setup documented for staticcheck dependency
• Real-world plugin functionality validated with chart diffing scenarios

Essential Context for Agents:

• Go 1.24.5 with Helm v3 requirements clearly specified
• Repository structure and key directories explained
• Environment variables for testing documented
• Cross-platform plugin installation process covered
• CI/CD workflow understanding for GitHub Actions

Comprehensive Validation Scenarios:
The instructions include step-by-step validation scenarios that agents should run after making changes:

• Basic plugin functionality testing
• Real chart diffing with test charts
• Plugin installation verification
• Complete development cycle validation

All instructions have been thoroughly tested to ensure they work correctly every time, following the requirement to validate every command before inclusion.

Fixes #837.

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