permissions: derive snapshot sandbox projections (#19775)

## Why

`ThreadConfigSnapshot` is used by app-server and thread metadata code as
a stable view of active runtime settings. Keeping both `sandbox_policy`
and `permission_profile` in the snapshot duplicates permission state and
makes it possible for the legacy projection to drift from the canonical
profile.

The legacy `sandbox` value is still needed at app-server compatibility
boundaries, so this PR derives it on demand from the snapshot profile
and cwd instead of storing it.

## What Changed

- Removes `ThreadConfigSnapshot.sandbox_policy`.
- Adds `ThreadConfigSnapshot::sandbox_policy()` as a compatibility
projection from `permission_profile` plus `cwd`.
- Updates app-server response/metadata code and tests to call the
projection only where legacy fields still exist.
- Keeps snapshot construction profile-only so split filesystem rules,
disabled enforcement, and external enforcement remain represented by the
canonical profile.

## Verification

- `cargo test -p codex-app-server
thread_response_permission_profile_preserves_enforcement --lib`
- `cargo test -p codex-core
dispatch_reclaims_stale_global_lock_and_starts_consolidation --lib`



































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[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/19775).
* #19900
* #19899
* #19776
* __->__ #19775

codex-rs/core/src/tools/handlers/multi_agents_tests.rs

@@ -2143,7 +2143,7 @@ async fn spawn_agent_reapplies_runtime_sandbox_after_role_config() {
         .expect("spawned agent thread should exist")
         .config_snapshot()
         .await;
-    assert_eq!(snapshot.sandbox_policy, expected_sandbox);
+    assert_eq!(snapshot.sandbox_policy(), expected_sandbox);
     assert_eq!(snapshot.approval_policy, AskForApproval::OnRequest);
     assert_eq!(snapshot.permission_profile, expected_permission_profile);
     let child_thread = manager