Include legacy deny paths in elevated Windows sandbox setup (#17365)

## Summary

This updates the Windows elevated sandbox setup/refresh path to include
the legacy `compute_allow_paths(...).deny` protected children in the
same deny-write payload pipe added for split filesystem carveouts.

Concretely, elevated setup and elevated refresh now both build
deny-write payload paths from:

- explicit split-policy deny-write paths, preserving missing paths so
setup can materialize them before applying ACLs
- legacy `compute_allow_paths(...).deny`, which includes existing
`.git`, `.codex`, and `.agents` children under writable roots

This lets the elevated backend protect `.git` consistently with the
unelevated/restricted-token path, and removes the old janky hard-coded
`.codex` / `.agents` elevated setup helpers in favor of the shared
payload path.

## Root Cause

The landed split-carveout PR threaded a `deny_write_paths` pipe through
elevated setup/refresh, but the legacy workspace-write deny set from
`compute_allow_paths(...).deny` was not included in that payload. As a
result, elevated workspace-write did not apply the intended deny-write
ACLs for existing protected children like `<cwd>/.git`.

## Notes

The legacy protected children still only enter the deny set if they
already exist, because `compute_allow_paths` filters `.git`, `.codex`,
and `.agents` with `exists()`. Missing explicit split-policy deny paths
are preserved separately because setup intentionally materializes those
before applying ACLs.

## Validation

- `cargo fmt --check -p codex-windows-sandbox`
- `cargo test -p codex-windows-sandbox`
- `cargo build -p codex-cli -p codex-windows-sandbox --bins`
- Elevated `codex exec` smoke with `windows.sandbox='elevated'`: fresh
git repo, attempted append to `.git/config`, observed `Access is
denied`, marker not written, Deny ACE present on `.git`
- Unelevated `codex exec` smoke with `windows.sandbox='unelevated'`:
fresh git repo, attempted append to `.git/config`, observed `Access is
denied`, marker not written, Deny ACE present on `.git`

codex-rs/windows-sandbox-rs/src/workspace_acl.rs

@@ -1,30 +1,6 @@
-use crate::acl::add_deny_write_ace;
 use crate::path_normalization::canonicalize_path;
-use anyhow::Result;
-use std::ffi::c_void;
 use std::path::Path;
 
 pub fn is_command_cwd_root(root: &Path, canonical_command_cwd: &Path) -> bool {
     canonicalize_path(root) == canonical_command_cwd
 }
-
-/// # Safety
-/// Caller must ensure `psid` is a valid SID pointer.
-pub unsafe fn protect_workspace_codex_dir(cwd: &Path, psid: *mut c_void) -> Result<bool> {
-    protect_workspace_subdir(cwd, psid, ".codex")
-}
-
-/// # Safety
-/// Caller must ensure `psid` is a valid SID pointer.
-pub unsafe fn protect_workspace_agents_dir(cwd: &Path, psid: *mut c_void) -> Result<bool> {
-    protect_workspace_subdir(cwd, psid, ".agents")
-}
-
-unsafe fn protect_workspace_subdir(cwd: &Path, psid: *mut c_void, subdir: &str) -> Result<bool> {
-    let path = cwd.join(subdir);
-    if path.is_dir() {
-        add_deny_write_ace(&path, psid)
-    } else {
-        Ok(false)
-    }
-}