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/core/src/mcp/skill_dependencies.rs

@@ -79,6 +79,7 @@ async fn should_install_mcp_dependencies(
             "The following MCP servers are required by the selected skills but are not installed yet: {server_list}. Install them now?"
         ),
         is_other: false,
+        is_secret: false,
         options: Some(vec![
             RequestUserInputQuestionOption {
                 label: MCP_DEPENDENCY_OPTION_INSTALL.to_string(),