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/open.rs

@@ -1,5 +1,7 @@
-use std::io;
-use std::process::{Command, Stdio};
+use std::{
+    io,
+    process::{Command, Stdio},
+};
 
 /// Kind of URL being opened (`apprt.action.OpenUrl.Kind`). On Linux the
 /// kind is ignored by [`open`] (it only matters for macOS `open -t`).