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/config.schema.json

@@ -207,6 +207,9 @@
             "shell_tool": {
               "type": "boolean"
             },
+            "skill_env_var_dependency_prompt": {
+              "type": "boolean"
+            },
             "skill_mcp_dependency_install": {
               "type": "boolean"
             },
@@ -1231,6 +1234,9 @@
         "shell_tool": {
           "type": "boolean"
         },
+        "skill_env_var_dependency_prompt": {
+          "type": "boolean"
+        },
         "skill_mcp_dependency_install": {
           "type": "boolean"
         },