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>

justfile

@@ -14,7 +14,7 @@ codex *args:
 exec *args:
     cargo run --bin codex -- exec "$@"
 
-# Start codex-exec-server and run codex-tui.
+# Start `codex exec-server` and run codex-tui.
 [no-cd]
 tui-with-exec-server *args:
     ./scripts/run_tui_with_exec_server.sh "$@"