Add environment variable to configure .claude config directory location

Users who work across multiple machines and use a portable home directory (e.g., a shared filesystem mounted at a consistent path) cannot easily relocate ~/.claude/ to their preferred location. An environment variable like CLAUDE_CONFIG_DIR (similar to how XDG_CONFIG_HOME works for other tools) would allow users to store Claude Code's config, credentials, history, and session data in a location of their choosing instead of being hardcoded to ~/.claude/.