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)
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* #19900
* #19899
* #19776
* __->__ #19775

codex-rs/core/src/session/session.rs

@@ -126,7 +126,6 @@ impl SessionConfiguration {
             service_tier: self.service_tier,
             approval_policy: self.approval_policy.value(),
             approvals_reviewer: self.approvals_reviewer,
-            sandbox_policy: self.sandbox_policy(),
             permission_profile: self.permission_profile(),
             cwd: self.cwd.clone(),
             ephemeral: self.original_config_do_not_use.ephemeral,