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/features.rs

@@ -115,6 +115,8 @@ pub enum Feature {
     Apps,
     /// Allow prompting and installing missing MCP dependencies.
     SkillMcpDependencyInstall,
+    /// Prompt for missing skill env var dependencies.
+    SkillEnvVarDependencyPrompt,
     /// Steer feature flag - when enabled, Enter submits immediately instead of queuing.
     Steer,
     /// Enable collaboration modes (Plan, Code, Pair Programming, Execute).
@@ -533,6 +535,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::SkillEnvVarDependencyPrompt,
+        key: "skill_env_var_dependency_prompt",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::Steer,
         key: "steer",