fix: make all fields of Session private (#2285)

As `Session` needs a bit of work, it will make things easier to move
around if we can start by reducing the extent of its public API. This
makes all the fields private, though adds three `pub(crate)` getters.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2285).
* #2287
* #2286
* __->__ #2285

codex-rs/core/src/apply_patch.rs

@@ -45,17 +45,13 @@ pub(crate) async fn apply_patch(
     call_id: &str,
     action: ApplyPatchAction,
 ) -> InternalApplyPatchInvocation {
-    let writable_roots_snapshot = {
-        #[allow(clippy::unwrap_used)]
-        let guard = sess.writable_roots.lock().unwrap();
-        guard.clone()
-    };
+    let writable_roots_snapshot = sess.get_writable_roots().to_vec();
 
     match assess_patch_safety(
         &action,
-        sess.approval_policy,
+        sess.get_approval_policy(),
         &writable_roots_snapshot,
-        &sess.cwd,
+        sess.get_cwd(),
     ) {
         SafetyCheck::AutoApprove { .. } => {
             InternalApplyPatchInvocation::DelegateToExec(ApplyPatchExec {