feat: Add plugin discovery and listing functionality

[yeoldegrove]

Add Plugin Discovery and Integration

This PR adds comprehensive plugin discovery and integration functionality to nilla-cli, making it easy for users to discover and use plugins like nilla-home and nilla-nixos.
This solves the issue #22.

Features

Plugin Discovery

• Automatically discovers nilla-* executables in the system PATH
• Caches plugin metadata (version, usage, commands, examples) to avoid redundant process calls

Plugin Listing

• New nilla plugins list command displays all available plugins in a formatted table
• Shows plugin name, path, version, completion support, and available commands

Help Integration

• Plugin information is automatically included in nilla --help output
• Displays usage, commands, and examples for each discovered plugin
• nilla <plugin> --help correctly shows the plugin's help instead of nilla's help

Usage: nilla [OPTIONS] [COMMAND]

[...]

Available Plugins
  home
    Usage: nilla home [OPTIONS] [COMMAND]
    Commands: switch, build, completions, help

  nixos
    Usage: nilla nixos [OPTIONS] [COMMAND]
    Commands: switch, test, build, completions, help

  Use `nilla plugins list` to see all plugins with details.
  Use `nilla <plugin> --help` to see plugin-specific help.

Shell Completions

• Plugins appear as top-level commands in shell completions (e.g., nilla <TAB> home <TAB>)
• Plugin commands are included in completions (e.g., nilla home switch <TAB>)
• Completions are generated by nilla-cli itself, ensuring consistent behavior across shells

nilla <TAB>
build        -- Build a package from a Nilla project
completions  -- Generate autocompletions for your shell
help         -- Print this message or the help of the given subcommand(s)
home         -- Plugin: /nix/store/465il15cv5x9bldi5pm0vnnj003b56p7-nilla-home-0.0.0/bin/nilla-home
nixos        -- Plugin: /nix/store/1nc4fk1x7inv8h4vzv167wvbdv5szyv2-nilla-nixos-0.0.0/bin/nilla-nixos
plugins      -- Manage and list available plugins
run          -- Run a package's main program
shell        -- Start a development shell from a Nilla project
show         -- Show information about a Nilla project

nilla nixos <TAB>
build              -- Build a NixOS system
completions        -- Generate autocompletions for your shell
help               -- Print this message or the help of the given subcommand(s)
switch             -- Build, install, and switch into a system
test               -- Test a system

Technical Details

• Plugin metadata is extracted by parsing --help and --version output
• Metadata parsing focuses on robust sections: "Usage:", "Commands:", and "Examples:"
• Plugin commands are dynamically added to the completion structure
• Help output transforms plugin usage from nilla-<plugin> to nilla <plugin> format