Refactor exec-server filesystem API into codex-file-system (#19892)

## Summary
- Extracted the shared filesystem types and `ExecutorFileSystem` trait
into a new `codex-file-system` crate
- Switched `codex-config` and `codex-git-utils` to depend on that crate
instead of `codex-exec-server`
- Kept `codex-exec-server` re-exporting the same API for existing
callers

## Testing
- Ran `cargo test -p codex-file-system`
- Ran `cargo test -p codex-git-utils`
- Ran `cargo test -p codex-config`
- Ran `cargo test -p codex-exec-server`
- Ran `just fix -p codex-file-system`, `just fix -p codex-git-utils`,
`just fix -p codex-config`, `just fix -p codex-exec-server`
- Ran `just fmt`
- Updated and verified the Bazel module lockfile

codex-rs/file-system/BUILD.bazel

@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "file-system",
+    crate_name = "codex_file_system",
+)