Review Mode (Core) (#3401)

## 📝 Review Mode -- Core

This PR introduces the Core implementation for Review mode:

- New op `Op::Review { prompt: String }:` spawns a child review task
with isolated context, a review‑specific system prompt, and a
`Config.review_model`.
- `EnteredReviewMode`: emitted when the child review session starts.
Every event from this point onwards reflects the review session.
- `ExitedReviewMode(Option<ReviewOutputEvent>)`: emitted when the review
finishes or is interrupted, with optional structured findings:

```json
{
  "findings": [
    {
      "title": "<≤ 80 chars, imperative>",
      "body": "<valid Markdown explaining *why* this is a problem; cite files/lines/functions>",
      "confidence_score": <float 0.0-1.0>,
      "priority": <int 0-3>,
      "code_location": {
        "absolute_file_path": "<file path>",
        "line_range": {"start": <int>, "end": <int>}
      }
    }
  ],
  "overall_correctness": "patch is correct" | "patch is incorrect",
  "overall_explanation": "<1-3 sentence explanation justifying the overall_correctness verdict>",
  "overall_confidence_score": <float 0.0-1.0>
}
```

## Questions

### Why separate out its own message history?

We want the review thread to match the training of our review models as
much as possible -- that means using a custom prompt, removing user
instructions, and starting a clean chat history.

We also want to make sure the review thread doesn't leak into the parent
thread.

### Why do this as a mode, vs. sub-agents?

1. We want review to be a synchronous task, so it's fine for now to do a
bespoke implementation.
2. We're still unclear about the final structure for sub-agents. We'd
prefer to land this quickly and then refactor into sub-agents without
rushing that implementation.

codex-rs/protocol/src/protocol.rs

@@ -166,6 +166,10 @@ pub enum Op {
     /// The agent will use its existing context (either conversation history or previous response id)
     /// to generate a summary which will be returned as an AgentMessage event.
     Compact,
+
+    /// Request a code review from the agent.
+    Review { review_request: ReviewRequest },
+
     /// Request to shut down codex instance.
     Shutdown,
 }
@@ -504,6 +508,12 @@ pub enum EventMsg {
     ShutdownComplete,
 
     ConversationPath(ConversationPathResponseEvent),
+
+    /// Entered review mode.
+    EnteredReviewMode(ReviewRequest),
+
+    /// Exited review mode with an optional final result to apply.
+    ExitedReviewMode(Option<ReviewOutputEvent>),
 }
 
 // Individual event payload types matching each `EventMsg` variant.
@@ -962,6 +972,57 @@ pub struct GitInfo {
     pub repository_url: Option<String>,
 }
 
+/// Review request sent to the review session.
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, TS)]
+pub struct ReviewRequest {
+    pub prompt: String,
+    pub user_facing_hint: String,
+}
+
+/// Structured review result produced by a child review session.
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, TS)]
+pub struct ReviewOutputEvent {
+    pub findings: Vec<ReviewFinding>,
+    pub overall_correctness: String,
+    pub overall_explanation: String,
+    pub overall_confidence_score: f32,
+}
+
+impl Default for ReviewOutputEvent {
+    fn default() -> Self {
+        Self {
+            findings: Vec::new(),
+            overall_correctness: String::default(),
+            overall_explanation: String::default(),
+            overall_confidence_score: 0.0,
+        }
+    }
+}
+
+/// A single review finding describing an observed issue or recommendation.
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, TS)]
+pub struct ReviewFinding {
+    pub title: String,
+    pub body: String,
+    pub confidence_score: f32,
+    pub priority: i32,
+    pub code_location: ReviewCodeLocation,
+}
+
+/// Location of the code related to a review finding.
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, TS)]
+pub struct ReviewCodeLocation {
+    pub absolute_file_path: PathBuf,
+    pub line_range: ReviewLineRange,
+}
+
+/// Inclusive line range in a file associated with the finding.
+#[derive(Debug, Clone, Deserialize, Serialize, PartialEq, TS)]
+pub struct ReviewLineRange {
+    pub start: u32,
+    pub end: u32,
+}
+
 #[derive(Debug, Clone, Deserialize, Serialize, TS)]
 pub struct ExecCommandBeginEvent {
     /// Identifier so this can be paired with the ExecCommandEnd event.