Run exec-server fs operations through sandbox helper (#17294)

## Summary
- run exec-server filesystem RPCs requiring sandboxing through a
`codex-fs` arg0 helper over stdin/stdout
- keep direct local filesystem execution for `DangerFullAccess` and
external sandbox policies
- remove the standalone exec-server binary path in favor of top-level
arg0 dispatch/runtime paths
- add sandbox escape regression coverage for local and remote filesystem
paths

## Validation
- `just fmt`
- `git diff --check`
- remote devbox: `cd codex-rs && bazel test --bes_backend=
--bes_results_url= //codex-rs/exec-server:all` (6/6 passed)

---------

Co-authored-by: Codex <noreply@openai.com>

scripts/run_tui_with_exec_server.sh

@@ -24,7 +24,7 @@ trap cleanup EXIT INT TERM HUP
 
 (
   cd "$cargo_root"
-  cargo run -p codex-exec-server -- --listen "$listen_url"
+  cargo run -p codex-cli --bin codex -- exec-server --listen "$listen_url"
 ) >"$stdout_log" 2>"$stderr_log" &
 server_pid="$!"
 
@@ -40,7 +40,7 @@ for _ in $(seq 1 "$((start_timeout_seconds * 20))"); do
   if ! kill -0 "$server_pid" >/dev/null 2>&1; then
     cat "$stderr_log" >&2 || true
     cat "$stdout_log" >&2 || true
-    echo "failed to start codex-exec-server" >&2
+    echo "failed to start codex exec-server" >&2
     exit 1
   fi
 
@@ -50,7 +50,7 @@ done
 if [[ -z "$exec_server_url" ]]; then
   cat "$stderr_log" >&2 || true
   cat "$stdout_log" >&2 || true
-  echo "timed out waiting ${start_timeout_seconds}s for codex-exec-server to report its websocket URL" >&2
+  echo "timed out waiting ${start_timeout_seconds}s for codex exec-server to report its websocket URL" >&2
   exit 1
 fi