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/state/session.rs

@@ -1,6 +1,7 @@
 //! Session-wide mutable state.
 
 use codex_protocol::models::ResponseItem;
+use std::collections::HashMap;
 use std::collections::HashSet;
 
 use crate::codex::SessionConfiguration;
@@ -16,6 +17,7 @@ pub(crate) struct SessionState {
     pub(crate) history: ContextManager,
     pub(crate) latest_rate_limits: Option<RateLimitSnapshot>,
     pub(crate) server_reasoning_included: bool,
+    pub(crate) dependency_env: HashMap<String, String>,
     pub(crate) mcp_dependency_prompted: HashSet<String>,
     /// Whether the session's initial context has been seeded into history.
     ///
@@ -33,6 +35,7 @@ impl SessionState {
             history,
             latest_rate_limits: None,
             server_reasoning_included: false,
+            dependency_env: HashMap::new(),
             mcp_dependency_prompted: HashSet::new(),
             initial_context_seeded: false,
         }
@@ -112,6 +115,16 @@ impl SessionState {
     pub(crate) fn mcp_dependency_prompted(&self) -> HashSet<String> {
         self.mcp_dependency_prompted.clone()
     }
+
+    pub(crate) fn set_dependency_env(&mut self, values: HashMap<String, String>) {
+        for (key, value) in values {
+            self.dependency_env.insert(key, value);
+        }
+    }
+
+    pub(crate) fn dependency_env(&self) -> HashMap<String, String> {
+        self.dependency_env.clone()
+    }
 }
 
 // Sometimes new snapshots don't include credits or plan information.