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/execpolicy/src/parser.rs

@@ -212,6 +212,7 @@ fn policy_builtins(builder: &mut GlobalsBuilder) {
         decision: Option<&'v str>,
         r#match: Option<UnpackList<Value<'v>>>,
         not_match: Option<UnpackList<Value<'v>>>,
+        justification: Option<&'v str>,
         eval: &mut Evaluator<'v, '_, '_>,
     ) -> anyhow::Result<NoneType> {
         let decision = match decision {
@@ -219,6 +220,14 @@ fn policy_builtins(builder: &mut GlobalsBuilder) {
             None => Decision::Allow,
         };
 
+        let justification = match justification {
+            Some(raw) if raw.trim().is_empty() => {
+                return Err(Error::InvalidRule("justification cannot be empty".to_string()).into());
+            }
+            Some(raw) => Some(raw.to_string()),
+            None => None,
+        };
+
         let pattern_tokens = parse_pattern(pattern)?;
 
         let matches: Vec<Vec<String>> =
@@ -246,6 +255,7 @@ fn policy_builtins(builder: &mut GlobalsBuilder) {
                         rest: rest.clone(),
                     },
                     decision,
+                    justification: justification.clone(),
                 }) as RuleRef
             })
             .collect();