windows-sandbox: drive write roots from resolved permissions (#22923)

## Why

This is the third PR in the Windows sandbox `SandboxPolicy` ->
`PermissionProfile` migration stack.

#22896 introduced `ResolvedWindowsSandboxPermissions`, and #22918 moved
elevated runner IPC to carry `PermissionProfile`. This PR starts moving
the remaining setup/spawn helpers away from asking legacy enum questions
like “is this `WorkspaceWrite`?” and toward resolved runtime permission
questions like “does this profile require write capability roots?”

## What changed

- Added resolved-permissions helpers for network identity and
write-capability detection.
- Moved setup write-root gathering to operate on
`ResolvedWindowsSandboxPermissions`, with the legacy `SandboxPolicy`
wrapper left in place for existing call sites.
- Updated identity setup, elevated capture setup, and world-writable
audit denies to use resolved write roots.
- Updated spawn preparation to carry resolved permissions in
`SpawnContext` and use them for network blocking, setup write roots,
elevated capability SID selection, and legacy capability roots.
- Removed a now-unused legacy write-root helper.

## Verification

- `cargo test -p codex-windows-sandbox`
- `just fix -p codex-windows-sandbox`
- Existing stack checks are green on #22896 and #22918; CI has started
for this PR.
















---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/22923).
* #23715
* #23714
* #23167
* __->__ #22923

codex-rs/windows-sandbox-rs/src/unified_exec/backends/legacy.rs

@@ -10,6 +10,7 @@ use crate::process::StdinMode;
 use crate::process::read_handle_loop;
 use crate::process::spawn_process_with_pipes;
 use crate::spawn_prep::LegacyAclSids;
+use crate::spawn_prep::SpawnPrepOptions;
 use crate::spawn_prep::allow_null_device_for_workspace_write;
 use crate::spawn_prep::apply_legacy_session_acl_rules;
 use crate::spawn_prep::legacy_session_capability_roots;
@@ -283,12 +284,15 @@ pub(crate) async fn spawn_windows_sandbox_session_legacy(
 ) -> Result<SpawnedProcess> {
     let common = prepare_legacy_spawn_context(
         policy_json_or_preset,
+        sandbox_policy_cwd,
         codex_home,
         cwd,
         &mut env_map,
         &command,
-        /*inherit_path*/ false,
-        /*add_git_safe_directory*/ false,
+        SpawnPrepOptions {
+            inherit_path: false,
+            add_git_safe_directory: false,
+        },
     )?;
     if !common.policy.has_full_disk_read_access() {
         anyhow::bail!("Restricted read-only access requires the elevated Windows sandbox backend");
@@ -311,7 +315,7 @@ pub(crate) async fn spawn_windows_sandbox_session_legacy(
     );
     let security =
         prepare_legacy_session_security(&common.policy, codex_home, cwd, capability_roots)?;
-    allow_null_device_for_workspace_write(common.is_workspace_write);
+    allow_null_device_for_workspace_write(common.uses_write_capabilities);
 
     apply_legacy_session_acl_rules(
         &common.policy,