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/rule.rs

@@ -63,6 +63,12 @@ pub enum RuleMatch {
         #[serde(rename = "matchedPrefix")]
         matched_prefix: Vec<String>,
         decision: Decision,
+        /// Optional rationale for why this rule exists.
+        ///
+        /// This can be supplied for any decision and may be surfaced in different contexts
+        /// (e.g., prompt reasons or rejection messages).
+        #[serde(skip_serializing_if = "Option::is_none")]
+        justification: Option<String>,
     },
     HeuristicsRuleMatch {
         command: Vec<String>,
@@ -83,6 +89,7 @@ impl RuleMatch {
 pub struct PrefixRule {
     pub pattern: PrefixPattern,
     pub decision: Decision,
+    pub justification: Option<String>,
 }
 
 pub trait Rule: Any + Debug + Send + Sync {
@@ -104,6 +111,7 @@ impl Rule for PrefixRule {
             .map(|matched_prefix| RuleMatch::PrefixRuleMatch {
                 matched_prefix,
                 decision: self.decision,
+                justification: self.justification.clone(),
             })
     }
 }