Add comprehensive GitHub Copilot instructions for helm-diff development

.github/copilot-instructions.md

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+# Helm Diff Plugin
+
+Helm Diff is a Go-based Helm plugin that provides diff functionality for comparing Helm charts and releases. It shows what changes would occur during helm upgrade, rollback, or between different releases/revisions.
+
+Always reference these instructions first and fallback to search or bash commands only when you encounter unexpected information that does not match the info here.
+
+## Working Effectively
+
+**Prerequisites:**
+- Go >= 1.21 (currently uses Go 1.24.5)
+- Helm v3 (tested with v3.17.4 and v3.18.6)
+- Make sure `/home/runner/go/bin` is in your PATH for staticcheck: `export PATH=$PATH:/home/runner/go/bin`
+
+**Bootstrap and Build Process:**
+- ALWAYS run: `make bootstrap` first - downloads dependencies and installs staticcheck. Takes <1 second (if already done) or ~50 seconds (first time).
+- Build the plugin: `make build` - includes linting and compiles the binary. Takes ~9 seconds after bootstrap.
+- NEVER CANCEL builds. Set timeout to 3+ minutes for bootstrap, 2+ minutes for build operations.
+
+**Testing:**
+- Run unit tests: `make test` - includes coverage analysis. Takes ~12 seconds. NEVER CANCEL - set timeout to 3+ minutes.
+- Tests include comprehensive coverage (38.7% overall) and use a fake helm binary for isolation.
+- Test coverage is generated in `cover.out` with detailed function-level coverage reports.
+
+**Linting and Code Quality:**
+- Local linting: `make lint` - runs gofmt, go vet, and staticcheck verification. Takes ~2 seconds.
+- Code formatting: `make format` - applies gofmt formatting automatically. Takes <1 second.
+- Full golangci-lint runs only in CI via GitHub Actions, not available locally.
+- ALWAYS run `make format` and `make lint` before committing changes.
+
+**Plugin Installation:**
+- Install as Helm plugin: `make install` or `make install/helm3` - builds and installs to Helm plugins directory. Takes ~3 seconds.
+- The plugin installs via `install-binary.sh` script which handles cross-platform binary installation.
+
+## Validation Scenarios
+
+**ALWAYS test your changes with these scenarios:**
+
+1. **Basic Plugin Functionality:**
+   ```bash
+   # Test the binary directly
+   ./bin/diff version
+   ./bin/diff --help
+   ./bin/diff upgrade --help
+   ```
+
+2. **Real Chart Diffing:**
+   ```bash
+   # Create a test chart and diff it
+   cd /tmp && helm create test-chart
+   cd /path/to/helm-diff
+   HELM_NAMESPACE=default HELM_BIN=helm ./bin/diff upgrade --install --dry-run test-release /tmp/test-chart
+   ```
+
+3. **Plugin Installation Verification:**
+   ```bash
+   # Test plugin installation
+   export HELM_DATA_HOME=/tmp/helm-test
+   make install
+   /tmp/helm-test/plugins/helm-diff/bin/diff version
+   ```
+
+## Build Times and Timeouts
+
+**CRITICAL: NEVER CANCEL long-running commands. Use these timeout values:**
+
+- `make bootstrap`: <1 second (if already done) or ~50 seconds (first time) (set timeout: 5+ minutes)
+- `make build`: ~9 seconds after bootstrap (set timeout: 3+ minutes)
+- `make test`: ~12 seconds (set timeout: 3+ minutes)
+- `make lint`: ~2 seconds (set timeout: 1 minute)
+- `make format`: <1 second (set timeout: 1 minute)
+- `make install`: ~3 seconds (set timeout: 2 minutes)
+
+## Common Tasks
+
+**Repository Structure:**
+- `main.go` - Entry point that delegates to cmd package
+- `cmd/` - Command-line interface implementation (upgrade, release, revision, rollback, version)
+- `diff/` - Core diffing logic and output formatting
+- `manifest/` - Kubernetes manifest parsing and handling
+- `scripts/` - Build and verification scripts (gofmt, govet, staticcheck)
+- `testdata/`, `*/testdata/` - Test fixtures and mock data
+- `plugin.yaml` - Helm plugin configuration
+- `install-binary.sh` - Cross-platform installation script
+- `Makefile` - Build system with all common targets
+
+**Key Files to Check After Changes:**
+- Always run tests after modifying `cmd/` or `diff/` packages
+- Check `plugin.yaml` version if making release changes
+- Verify `Makefile` targets if changing build process
+- Review `install-binary.sh` if modifying installation process
+
+**Environment Variables for Testing:**
+- `HELM_NAMESPACE` - Kubernetes namespace for operations
+- `HELM_BIN` - Path to helm binary (for direct testing)
+- `HELM_DIFF_USE_UPGRADE_DRY_RUN` - Use helm upgrade --dry-run instead of template
+- `HELM_DIFF_THREE_WAY_MERGE` - Enable three-way merge diffing
+- `HELM_DIFF_NORMALIZE_MANIFESTS` - Normalize YAML before diffing
+- `HELM_DIFF_OUTPUT_CONTEXT` - Configure output context lines
+
+**CI/CD Information:**
+- GitHub Actions runs on push/PR to master branch
+- Tests run on Ubuntu, macOS, Windows with multiple Helm versions
+- Integration tests use Kind (Kubernetes in Docker)
+- Linting uses golangci-lint via GitHub Actions (not available locally)
+- Cross-platform plugin installation is tested via Docker
+
+**Direct Binary Usage (for development):**
+```bash
+# Build and test directly without Helm plugin installation
+go build -o bin/diff -ldflags="-X github.com/databus23/helm-diff/v3/cmd.Version=dev"
+HELM_NAMESPACE=default HELM_BIN=helm ./bin/diff upgrade --install --dry-run my-release ./chart-path
+```
+
+**Common Commands Reference:**
+```bash
+# Full development cycle
+make bootstrap      # Install dependencies (once)
+make build         # Build with linting  
+make test          # Run all tests
+make format        # Format code
+make install       # Install as Helm plugin
+
+# Validation
+./bin/diff version                    # Check version
+./bin/diff upgrade --help            # Check help
+make test                           # Run test suite
+```
+
+## Important Notes
+
+- The plugin supports Helm v3 only (v2 support was removed)
+- Uses Go modules for dependency management
+- Cross-platform support: Linux, macOS, Windows, FreeBSD
+- Multiple output formats: diff, simple, template, dyff
+- Supports both client-side and server-side dry-run modes
+- Includes three-way merge capabilities for advanced diffing
+- Plugin binary is named `diff` and installed as `helm diff` command