feat: add justification arg to prefix_rule() in *.rules (#8751)

Adds an optional `justification` parameter to the `prefix_rule()`
execpolicy DSL so policy authors can attach human-readable rationale to
a rule. That justification is propagated through parsing/matching and
can be surfaced to the model (or approval UI) when a command is blocked
or requires approval.

When a command is rejected (or gated behind approval) due to policy, a
generic message makes it hard for the model/user to understand what went
wrong and what to do instead. Allowing policy authors to supply a short
justification improves debuggability and helps guide the model toward
compliant alternatives.

Example:

```python
prefix_rule(
    pattern = ["git", "push"],
    decision = "forbidden",
    justification = "pushing is blocked in this repo",
)
```

If Codex tried to run `git push origin main`, now the failure would
include:

```
`git push origin main` rejected: pushing is blocked in this repo
```

whereas previously, all it was told was:

```
execpolicy forbids this command
```

codex-rs/cli/tests/execpolicy.rs

@@ -59,3 +59,61 @@ prefix_rule(
 
     Ok(())
 }
+
+#[test]
+fn execpolicy_check_includes_justification_when_present() -> Result<(), Box<dyn std::error::Error>>
+{
+    let codex_home = TempDir::new()?;
+    let policy_path = codex_home.path().join("rules").join("policy.rules");
+    fs::create_dir_all(
+        policy_path
+            .parent()
+            .expect("policy path should have a parent"),
+    )?;
+    fs::write(
+        &policy_path,
+        r#"
+prefix_rule(
+    pattern = ["git", "push"],
+    decision = "forbidden",
+    justification = "pushing is blocked in this repo",
+)
+"#,
+    )?;
+
+    let output = Command::new(codex_utils_cargo_bin::cargo_bin("codex")?)
+        .env("CODEX_HOME", codex_home.path())
+        .args([
+            "execpolicy",
+            "check",
+            "--rules",
+            policy_path
+                .to_str()
+                .expect("policy path should be valid UTF-8"),
+            "git",
+            "push",
+            "origin",
+            "main",
+        ])
+        .output()?;
+
+    assert!(output.status.success());
+    let result: serde_json::Value = serde_json::from_slice(&output.stdout)?;
+    assert_eq!(
+        result,
+        json!({
+            "decision": "forbidden",
+            "matchedRules": [
+                {
+                    "prefixRuleMatch": {
+                        "matchedPrefix": ["git", "push"],
+                        "decision": "forbidden",
+                        "justification": "pushing is blocked in this repo"
+                    }
+                }
+            ]
+        })
+    );
+
+    Ok(())
+}