chore: sanbox extraction (#4286)

# Extract and Centralize Sandboxing
- Goal: Improve safety and clarity by centralizing sandbox planning and
execution.
  - Approach:
- Add planner (ExecPlan) and backend registry (Direct/Seatbelt/Linux)
with run_with_plan.
- Refactor codex.rs to plan-then-execute; handle failures/escalation via
the plan.
- Delegate apply_patch to the codex binary and run it with an empty env
for determinism.

codex-rs/core/src/executor/cache.rs

@@ -0,0 +1,51 @@
+use std::collections::HashSet;
+use std::sync::Arc;
+use std::sync::Mutex;
+
+#[derive(Clone, Debug, Default)]
+/// Thread-safe store of user approvals so repeated commands can reuse
+/// previously granted trust.
+pub(crate) struct ApprovalCache {
+    inner: Arc<Mutex<HashSet<Vec<String>>>>,
+}
+
+impl ApprovalCache {
+    pub(crate) fn insert(&self, command: Vec<String>) {
+        if command.is_empty() {
+            return;
+        }
+        if let Ok(mut guard) = self.inner.lock() {
+            guard.insert(command);
+        }
+    }
+
+    pub(crate) fn snapshot(&self) -> HashSet<Vec<String>> {
+        self.inner.lock().map(|g| g.clone()).unwrap_or_default()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn insert_ignores_empty_and_dedupes() {
+        let cache = ApprovalCache::default();
+
+        // Empty should be ignored
+        cache.insert(vec![]);
+        assert!(cache.snapshot().is_empty());
+
+        // Insert a command and verify snapshot contains it
+        let cmd = vec!["foo".to_string(), "bar".to_string()];
+        cache.insert(cmd.clone());
+        let snap1 = cache.snapshot();
+        assert!(snap1.contains(&cmd));
+
+        // Reinserting should not create duplicates
+        cache.insert(cmd);
+        let snap2 = cache.snapshot();
+        assert_eq!(snap1, snap2);
+    }
+}