chore: upgrade to Rust 1.89 (#2465)

Codex created this PR from the following prompt:

> upgrade this entire repo to Rust 1.89. Note that this requires
updating codex-rs/rust-toolchain.toml as well as the workflows in
.github/. Make sure that things are "clippy clean" as this change will
likely uncover new Clippy errors. `just fmt` and `cargo clippy --tests`
are sufficient to check for correctness

Note this modifies a lot of lines because it folds nested `if`
statements using `&&`.

---
[//]: # (BEGIN SAPLING FOOTER)
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* #2467
* __->__ #2465

codex-rs/tui/src/exec_command.rs

@@ -33,10 +33,10 @@ where
         return None;
     }
 
-    if let Some(home_dir) = std::env::var_os("HOME").map(PathBuf::from) {
-        if let Ok(rel) = path.strip_prefix(&home_dir) {
-            return Some(rel.to_path_buf());
-        }
+    if let Some(home_dir) = std::env::var_os("HOME").map(PathBuf::from)
+        && let Ok(rel) = path.strip_prefix(&home_dir)
+    {
+        return Some(rel.to_path_buf());
     }
 
     None