fix: writable_roots doesn't recognize home directory symbol in non-windows OS (#9193)

Fixes:
```
[sandbox_workspace_write]
writable_roots = ["~/code/"]
```

translates to
```
/Users/ccunningham/.codex/~/code
```
(i.e. the home dir symbol isn't recognized)

codex-rs/utils/absolute-path/src/lib.rs

@@ -1,3 +1,4 @@
+use dirs::home_dir;
 use path_absolutize::Absolutize;
 use schemars::JsonSchema;
 use serde::Deserialize;
@@ -21,16 +22,35 @@ use ts_rs::TS;
 pub struct AbsolutePathBuf(PathBuf);
 
 impl AbsolutePathBuf {
+    fn maybe_expand_home_directory(path: &Path) -> PathBuf {
+        let Some(path_str) = path.to_str() else {
+            return path.to_path_buf();
+        };
+        if cfg!(not(target_os = "windows"))
+            && let Some(home) = home_dir()
+        {
+            if path_str == "~" {
+                return home;
+            }
+            if let Some(rest) = path_str.strip_prefix("~/") {
+                return home.join(rest);
+            }
+        }
+        path.to_path_buf()
+    }
+
     pub fn resolve_path_against_base<P: AsRef<Path>, B: AsRef<Path>>(
         path: P,
         base_path: B,
     ) -> std::io::Result<Self> {
-        let absolute_path = path.as_ref().absolutize_from(base_path.as_ref())?;
+        let expanded = Self::maybe_expand_home_directory(path.as_ref());
+        let absolute_path = expanded.absolutize_from(base_path.as_ref())?;
         Ok(Self(absolute_path.into_owned()))
     }
 
     pub fn from_absolute_path<P: AsRef<Path>>(path: P) -> std::io::Result<Self> {
-        let absolute_path = path.as_ref().absolutize()?;
+        let expanded = Self::maybe_expand_home_directory(path.as_ref());
+        let absolute_path = expanded.absolutize()?;
         Ok(Self(absolute_path.into_owned()))
     }
 
@@ -165,6 +185,7 @@ impl<'de> Deserialize<'de> for AbsolutePathBuf {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use pretty_assertions::assert_eq;
     use tempfile::tempdir;
 
     #[test]
@@ -203,4 +224,47 @@ mod tests {
             base_dir.join(relative_path).as_path()
         );
     }
+
+    #[cfg(not(target_os = "windows"))]
+    #[test]
+    fn home_directory_root_on_non_windows_is_expanded_in_deserialization() {
+        let Some(home) = home_dir() else {
+            return;
+        };
+        let temp_dir = tempdir().expect("base dir");
+        let abs_path_buf = {
+            let _guard = AbsolutePathBufGuard::new(temp_dir.path());
+            serde_json::from_str::<AbsolutePathBuf>("\"~\"").expect("failed to deserialize")
+        };
+        assert_eq!(abs_path_buf.as_path(), home.as_path());
+    }
+
+    #[cfg(not(target_os = "windows"))]
+    #[test]
+    fn home_directory_subpath_on_non_windows_is_expanded_in_deserialization() {
+        let Some(home) = home_dir() else {
+            return;
+        };
+        let temp_dir = tempdir().expect("base dir");
+        let abs_path_buf = {
+            let _guard = AbsolutePathBufGuard::new(temp_dir.path());
+            serde_json::from_str::<AbsolutePathBuf>("\"~/code\"").expect("failed to deserialize")
+        };
+        assert_eq!(abs_path_buf.as_path(), home.join("code").as_path());
+    }
+
+    #[cfg(target_os = "windows")]
+    #[test]
+    fn home_directory_on_windows_is_not_expanded_in_deserialization() {
+        let temp_dir = tempdir().expect("base dir");
+        let base_dir = temp_dir.path();
+        let abs_path_buf = {
+            let _guard = AbsolutePathBufGuard::new(base_dir);
+            serde_json::from_str::<AbsolutePathBuf>("\"~/code\"").expect("failed to deserialize")
+        };
+        assert_eq!(
+            abs_path_buf.as_path(),
+            base_dir.join("~").join("code").as_path()
+        );
+    }
 }