Parse out frontmatter for custom prompts (#4456)

[Cherry picked from https://github.com/openai/codex/pull/3565]

Removes the frontmatter description/args from custom prompt files and
only includes body.

codex-rs/protocol/src/custom_prompts.rs

@@ -8,4 +8,6 @@ pub struct CustomPrompt {
     pub name: String,
     pub path: PathBuf,
     pub content: String,
+    pub description: Option<String>,
+    pub argument_hint: Option<String>,
 }