Reuse guardian session across approvals (#14668)

## Summary
- reuse a guardian subagent session across approvals so reviews keep a
stable prompt cache key and avoid one-shot startup overhead
- clear the guardian child history before each review so prior guardian
decisions do not leak into later approvals
- include the `smart_approvals` -> `guardian_approval` feature flag
rename in the same PR to minimize release latency on a very tight
timeline
- add regression coverage for prompt-cache-key reuse without
prior-review prompt bleed

## Request
- Bug/enhancement request: internal guardian prompt-cache and latency
improvement request

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/config.schema.json

@@ -395,6 +395,9 @@
             "fast_mode": {
               "type": "boolean"
             },
+            "guardian_approval": {
+              "type": "boolean"
+            },
             "image_detail_original": {
               "type": "boolean"
             },
@@ -473,9 +476,6 @@
             "skill_mcp_dependency_install": {
               "type": "boolean"
             },
-            "smart_approvals": {
-              "type": "boolean"
-            },
             "sqlite": {
               "type": "boolean"
             },
@@ -1944,6 +1944,9 @@
         "fast_mode": {
           "type": "boolean"
         },
+        "guardian_approval": {
+          "type": "boolean"
+        },
         "image_detail_original": {
           "type": "boolean"
         },
@@ -2022,9 +2025,6 @@
         "skill_mcp_dependency_install": {
           "type": "boolean"
         },
-        "smart_approvals": {
-          "type": "boolean"
-        },
         "sqlite": {
           "type": "boolean"
         },