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/app-server-protocol/src/protocol/v2.rs

@@ -2532,6 +2532,8 @@ pub struct ToolRequestUserInputQuestion {
     pub question: String,
     #[serde(default)]
     pub is_other: bool,
+    #[serde(default)]
+    pub is_secret: bool,
     pub options: Option<Vec<ToolRequestUserInputOption>>,
 }