Honor XDG_CONFIG_HOME for config discovery

[ychampion]

Summary

• resolve the default config directory from XDG_CONFIG_HOME when it is an absolute path
• retain ~/.dust.toml and ~/.config/dust/config.toml compatibility
• cover custom and fallback config locations with a unit test

Why

Dust currently ignores $XDG_CONFIG_HOME, so users with a custom XDG config directory must pass --config on every invocation.

Closes #577

Validation

• cargo test --bin dust
• cargo test --test test_flags
• cargo test --test tests_symlinks
• cargo test --test test_exact_output -- --skip test_permission_normal --skip test_permission_flag
• cargo fmt --check
• live CLI smoke with an XDG config enabling JSON output