Refactor: Migrate to clap for CLI argument parsing

[devark28]

Overview

This PR replaces the custom CLI argument parsing implementation with clap, a robust and well-maintained argument parsing library. This change improves maintainability, user experience, and reduces the codebase complexity.

Key Changes

1. CLI Parsing Migration

• Removed custom argument parsing logic including:

  • CliCmd trait and all command-specific parsing implementations
  • try_parse_cmd! macro for command matching
  • Individual command structs (FormatCmd, LintCmd, ListCmd, PickCmd, HelpCmd, VersionCmd)

• Added clap-based argument parsing:

  • New Args struct with derive macros for declarative CLI definition
  • ArgCommands enum for subcommand handling
  • Automatic help generation and validation

2. Improved User Experience

• Enhanced help text with detailed usage examples and input mode explanations
• Support for --version / -v flag (in addition to version subcommand)
• Better error messages through clap's built-in validation
• Consistent command interface with clear subcommand structure

3. Code Simplification

• Removed ~400 lines of custom parsing logic
• Consolidated command handling into a single, maintainable module (cli/args.rs)
• Simplified Cli::init() method with cleaner input resolution logic
• Removed unnecessary command-specific modules and their implementations

4. Module Restructuring

• Moved engine command implementations from engine/commands/ to engine/ root
  • engine/commands/format.rs → engine/format.rs
  • engine/commands/list.rs → engine/list.rs
  • engine/commands/pick.rs → engine/pick.rs
  • engine/commands/version.rs → engine/version.rs
• Removed engine/commands/help.rs (now handled by clap)

5. Code Cleanup

• Removed unused methods and error variants:
  • AccessErrors::VariableNotFound
  • CliErrors::UnknownCommand and CliErrors::FailedToParseArgs
  • ParsingErrors::BrokenValidator
  • NamingErrors unused warnings
  • Various unused helper methods in Parser, Document, Block, and Line
• Cleaned up redundant test code using matches! macro

Dependencies

• Added clap = { version = "4.0", features = ["derive"] } to Cargo.toml

Behavioral Changes

• Help is now shown via envmn --help or envmn help (clap standard)
• Version can be shown via envmn --version / -v or envmn version
• Error messages are now more descriptive and consistent with clap conventions
• Default file (.env) behavior remains unchanged

Testing

• All existing tests pass
• Manual testing confirms:
  • All subcommands work as expected
  • Stdin piping continues to work correctly
  • File input resolution follows the documented priority (piped input > explicit file > default .env)

Migration Notes

This is a refactoring PR with no breaking changes to the public CLI interface. All existing commands and workflows continue to work as before, but with improved error messages and help text.