style: apply nightly rustfmt import grouping across the workspace

rustfmt.toml sets group_imports, imports_granularity and imports_layout,
which are nightly-only; agents formatted with stable rustfmt, which
ignored them. This is a formatting-only pass with `cargo +nightly fmt`.

Test Plan:
- cargo clippy --workspace --all-targets -- -D warnings
- cargo test --workspace

crates/ghostty-cli/src/validate_config.rs

@@ -2,8 +2,7 @@
 
 use std::io::Write;
 
-use ghostty_config::config::Config;
-use ghostty_config::config_struct;
+use ghostty_config::{config::Config, config_struct};
 
 use crate::error::Error;