Use AbsolutePathBuf in skill loading and codex_home (#17407)

Helps with FS migration later

codex-rs/protocol/src/protocol.rs

@@ -3332,7 +3332,7 @@ pub struct SkillMetadata {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     #[ts(optional)]
     pub dependencies: Option<SkillDependencies>,
-    pub path: PathBuf,
+    pub path: AbsolutePathBuf,
     pub scope: SkillScope,
     pub enabled: bool,
 }