Add environment variable support for global arguments

[Copilot]

Enables clap's env feature and adds environment variable mappings for all global CLI arguments.

Changes

• Added env feature to clap dependency
• Mapped global arguments to environment variables using DUA_ prefix:
  • DUA_THREADS → --threads
  • DUA_FORMAT → --format
  • DUA_APPARENT_SIZE → --apparent-size
  • DUA_COUNT_HARD_LINKS → --count-hard-links
  • DUA_STAY_ON_FILESYSTEM → --stay-on-filesystem
  • DUA_IGNORE_DIRS → --ignore-dirs
  • DUA_LOG_FILE → --log-file

Usage

# Set default format
export DUA_FORMAT=metric
dua

# CLI args override env vars
DUA_FORMAT=bytes dua --format metric  # Uses metric

Environment variables are documented in --help output and work across all subcommands.

Original prompt

Add support for environment variables for all global arguments. This needs the env feature to be enabled in clap.

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[Byron]

Perfect!