Add code_mode_only feature (#14617)

Summary
- add the code_mode_only feature flag/config schema and wire its
dependency on code_mode
- update code mode tool descriptions to list nested tools with detailed
headers
- restrict available tools for prompt and exec descriptions when
code_mode_only is enabled and test the behavior

Testing
- Not run (not requested)

codex-rs/core/config.schema.json

@@ -339,6 +339,9 @@
             "code_mode": {
               "type": "boolean"
             },
+            "code_mode_only": {
+              "type": "boolean"
+            },
             "codex_git_commit": {
               "type": "boolean"
             },
@@ -1880,6 +1883,9 @@
         "code_mode": {
           "type": "boolean"
         },
+        "code_mode_only": {
+          "type": "boolean"
+        },
         "codex_git_commit": {
           "type": "boolean"
         },