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/src/config/mod.rs

@@ -304,6 +304,9 @@ pub struct Config {
     /// When this program is invoked, arg0 will be set to `codex-linux-sandbox`.
     pub codex_linux_sandbox_exe: Option<PathBuf>,
 
+    /// Optional absolute path to the Node runtime used by `js_repl`.
+    pub js_repl_node_path: Option<PathBuf>,
+
     /// Value to use for `reasoning.effort` when making a request using the
     /// Responses API.
     pub model_reasoning_effort: Option<ReasoningEffort>,
@@ -939,6 +942,9 @@ pub struct ConfigToml {
     /// Token budget applied when storing tool/function outputs in the context manager.
     pub tool_output_token_limit: Option<usize>,
 
+    /// Optional absolute path to the Node runtime used by `js_repl`.
+    pub js_repl_node_path: Option<AbsolutePathBuf>,
+
     /// Profile to use from the `profiles` map.
     pub profile: Option<String>,
 
@@ -1282,6 +1288,7 @@ pub struct ConfigOverrides {
     pub model_provider: Option<String>,
     pub config_profile: Option<String>,
     pub codex_linux_sandbox_exe: Option<PathBuf>,
+    pub js_repl_node_path: Option<PathBuf>,
     pub base_instructions: Option<String>,
     pub developer_instructions: Option<String>,
     pub personality: Option<Personality>,
@@ -1408,6 +1415,7 @@ impl Config {
             model_provider,
             config_profile: config_profile_key,
             codex_linux_sandbox_exe,
+            js_repl_node_path: js_repl_node_path_override,
             base_instructions,
             developer_instructions,
             personality,
@@ -1641,6 +1649,9 @@ impl Config {
             "experimental compact prompt file",
         )?;
         let compact_prompt = compact_prompt.or(file_compact_prompt);
+        let js_repl_node_path = js_repl_node_path_override
+            .or(config_profile.js_repl_node_path.map(Into::into))
+            .or(cfg.js_repl_node_path.map(Into::into));
 
         let review_model = override_review_model.or(cfg.review_model);
 
@@ -1757,6 +1768,7 @@ impl Config {
             ephemeral: ephemeral.unwrap_or_default(),
             file_opener: cfg.file_opener.unwrap_or(UriBasedFileOpener::VsCode),
             codex_linux_sandbox_exe,
+            js_repl_node_path,
 
             hide_agent_reasoning: cfg.hide_agent_reasoning.unwrap_or(false),
             show_raw_agent_reasoning: cfg
@@ -4026,6 +4038,7 @@ model_verbosity = "high"
                 ephemeral: false,
                 file_opener: UriBasedFileOpener::VsCode,
                 codex_linux_sandbox_exe: None,
+                js_repl_node_path: None,
                 hide_agent_reasoning: false,
                 show_raw_agent_reasoning: false,
                 model_reasoning_effort: Some(ReasoningEffort::High),
@@ -4132,6 +4145,7 @@ model_verbosity = "high"
             ephemeral: false,
             file_opener: UriBasedFileOpener::VsCode,
             codex_linux_sandbox_exe: None,
+            js_repl_node_path: None,
             hide_agent_reasoning: false,
             show_raw_agent_reasoning: false,
             model_reasoning_effort: None,
@@ -4236,6 +4250,7 @@ model_verbosity = "high"
             ephemeral: false,
             file_opener: UriBasedFileOpener::VsCode,
             codex_linux_sandbox_exe: None,
+            js_repl_node_path: None,
             hide_agent_reasoning: false,
             show_raw_agent_reasoning: false,
             model_reasoning_effort: None,
@@ -4326,6 +4341,7 @@ model_verbosity = "high"
             ephemeral: false,
             file_opener: UriBasedFileOpener::VsCode,
             codex_linux_sandbox_exe: None,
+            js_repl_node_path: None,
             hide_agent_reasoning: false,
             show_raw_agent_reasoning: false,
             model_reasoning_effort: Some(ReasoningEffort::High),