Better handling skill depdenencies on ENV VAR. (#9017)

An experimental flow for env var skill dependencies. Skills can now
declare required env vars in SKILL.md; if missing, the CLI prompts the
user to get the value, and Core will store it in memory (eventually to a
local persistent store)
<img width="790" height="169" alt="image"
src="https://github.com/user-attachments/assets/cd928918-9403-43cb-a7e7-b8d59bcccd9a"
/>

codex-rs/protocol/src/request_user_input.rs

@@ -20,6 +20,10 @@ pub struct RequestUserInputQuestion {
     #[schemars(rename = "isOther")]
     #[ts(rename = "isOther")]
     pub is_other: bool,
+    #[serde(rename = "isSecret", default)]
+    #[schemars(rename = "isSecret")]
+    #[ts(rename = "isSecret")]
+    pub is_secret: bool,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub options: Option<Vec<RequestUserInputQuestionOption>>,
 }