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/tools/handlers/shell.rs

@@ -233,6 +233,12 @@ impl ShellHandler {
             None
         };
 
+        let mut exec_params = exec_params;
+        let dependency_env = session.dependency_env().await;
+        if !dependency_env.is_empty() {
+            exec_params.env.extend(dependency_env);
+        }
+
         // Approval policy guard for explicit escalation in non-OnRequest modes.
         if exec_params
             .sandbox_permissions