execpolicy helpers (#7032)

this PR 
- adds a helper function to amend `.codexpolicy` files with new prefix
rules
- adds a utility to `Policy` allowing prefix rules to be added to
existing `Policy` structs

both additions will be helpful as we thread codexpolicy into the TUI
workflow

codex-rs/execpolicy/src/policy.rs

@@ -1,9 +1,15 @@
 use crate::decision::Decision;
+use crate::error::Error;
+use crate::error::Result;
+use crate::rule::PatternToken;
+use crate::rule::PrefixPattern;
+use crate::rule::PrefixRule;
 use crate::rule::RuleMatch;
 use crate::rule::RuleRef;
 use multimap::MultiMap;
 use serde::Deserialize;
 use serde::Serialize;
+use std::sync::Arc;
 
 #[derive(Clone, Debug)]
 pub struct Policy {
@@ -23,6 +29,27 @@ impl Policy {
         &self.rules_by_program
     }
 
+    pub fn add_prefix_rule(&mut self, prefix: &[String], decision: Decision) -> Result<()> {
+        let (first_token, rest) = prefix
+            .split_first()
+            .ok_or_else(|| Error::InvalidPattern("prefix cannot be empty".to_string()))?;
+
+        let rule: RuleRef = Arc::new(PrefixRule {
+            pattern: PrefixPattern {
+                first: Arc::from(first_token.as_str()),
+                rest: rest
+                    .iter()
+                    .map(|token| PatternToken::Single(token.clone()))
+                    .collect::<Vec<_>>()
+                    .into(),
+            },
+            decision,
+        });
+
+        self.rules_by_program.insert(first_token.clone(), rule);
+        Ok(())
+    }
+
     pub fn check(&self, cmd: &[String]) -> Evaluation {
         let rules = match cmd.first() {
             Some(first) => match self.rules_by_program.get_vec(first) {