Add feature-gated freeform js_repl core runtime (#10674)

## Summary

This PR adds an **experimental, feature-gated `js_repl` core runtime**
so models can execute JavaScript in a persistent REPL context across
tool calls.

The implementation integrates with existing feature gating, tool
registration, prompt composition, config/schema docs, and tests.

## What changed

- Added new experimental feature flag: `features.js_repl`.
- Added freeform `js_repl` tool and companion `js_repl_reset` tool.
- Gated tool availability behind `Feature::JsRepl`.
- Added conditional prompt-section injection for JS REPL instructions
via marker-based prompt processing.
- Implemented JS REPL handlers, including freeform parsing and pragma
support (timeout/reset controls).
- Added runtime resolution order for Node:
  1. `CODEX_JS_REPL_NODE_PATH`
  2. `js_repl_node_path` in config
  3. `PATH`
- Added JS runtime assets/version files and updated docs/schema.

## Why

This enables richer agent workflows that require incremental JavaScript
execution with preserved state, while keeping rollout safe behind an
explicit feature flag.

## Testing

Coverage includes:

- Feature-flag gating behavior for tool exposure.
- Freeform parser/pragma handling edge cases.
- Runtime behavior (state persistence across calls and top-level `await`
support).

## Usage

```toml
[features]
js_repl = true
```

Optional runtime override:

- `CODEX_JS_REPL_NODE_PATH`, or
- `js_repl_node_path` in config.

#### [git stack](https://github.com/magus/git-stack-cli)
- 👉 `1` https://github.com/openai/codex/pull/10674
- ⏳ `2` https://github.com/openai/codex/pull/10672
- ⏳ `3` https://github.com/openai/codex/pull/10671
- ⏳ `4` https://github.com/openai/codex/pull/10673
- ⏳ `5` https://github.com/openai/codex/pull/10670

codex-rs/core/config.schema.json

@@ -221,6 +221,9 @@
             "include_apply_patch_tool": {
               "type": "boolean"
             },
+            "js_repl": {
+              "type": "boolean"
+            },
             "memory_tool": {
               "type": "boolean"
             },
@@ -290,6 +293,9 @@
         "include_apply_patch_tool": {
           "type": "boolean"
         },
+        "js_repl_node_path": {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        },
         "model": {
           "type": "string"
         },
@@ -1299,6 +1305,9 @@
         "include_apply_patch_tool": {
           "type": "boolean"
         },
+        "js_repl": {
+          "type": "boolean"
+        },
         "memory_tool": {
           "type": "boolean"
         },
@@ -1421,6 +1430,14 @@
       "description": "System instructions.",
       "type": "string"
     },
+    "js_repl_node_path": {
+      "allOf": [
+        {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        }
+      ],
+      "description": "Optional absolute path to the Node runtime used by `js_repl`."
+    },
     "log_dir": {
       "allOf": [
         {