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-os/src/shell.rs

@@ -1,5 +1,7 @@
-use std::ffi::{CString, NulError};
-use std::io::{self, Write};
+use std::{
+    ffi::{CString, NulError},
+    io::{self, Write},
+};
 
 /// Builder for constructing space-separated shell command strings.
 #[derive(Debug, Default, Clone)]