Implement EnvironmentManager for safe environment variable and PATH management

[gnodet]

• Create EnvironmentManager class with safe environment variable management
• Add special PATH handling with AddToPath() and AppendToPath() methods
• Update EnvironmentProvider interface to use EnvironmentManager instead of map
• Update all tools (Go, Java, Maven, Mvnd, Node) to use new EnvironmentProvider interface
• Implement comprehensive Go environment setup with GOROOT, GOPATH, and PATH management
• Add EnvGoPath constant for GOPATH environment variable
• Fix executor environment setup to properly merge tool-specific PATH modifications
• Resolve golangci-lint PATH issues by ensuring GOPATH/bin is included in PATH
• Use 'env golangci-lint run' workaround for mvx-shell interpreter limitations
• Make Manager.SetupEnvironment() the single source of truth for all environment setup
• Remove duplicated PATH logic from cmd/env.go and cmd/root.go
• Simplify executor to use complete environment from Manager
• Manager now starts with system environment and adds tool-specific modifications
• Each tool contributes both HOME directories and PATH entries via SetupEnvironment()
• Eliminates scattered environment logic across multiple files
• Follows Single Responsibility Principle with centralized environment management

This establishes a robust foundation for environment management across all tools
and resolves the core issue where tool-specific PATH modifications were being
lost during command execution. It also resolves the architectural issue where environment
setup was scattered across multiple files with inconsistent behavior between shell
integration, global setup, and command execution.