exec-server: add environment path refs (#25121)

## Summary
- add public `codex_exec_server::EnvironmentPathRef`
- bind an absolute path to its owning executor filesystem
- keep path operations in the next review slice

## Stack
- 1/5 in the skills path authority stack extracted from
https://github.com/openai/codex/pull/25098

## Validation
- `cd /Users/starr/code/codex-worktrees/pr-25098-restack4/codex-rs &&
just fmt`
- GitHub CI pending on rewritten head

codex-rs/exec-server/src/lib.rs

@@ -3,6 +3,7 @@ mod client_api;
 mod client_transport;
 mod connection;
 mod environment;
+mod environment_path;
 mod environment_provider;
 mod environment_toml;
 mod fs_helper;
@@ -43,6 +44,7 @@ pub use environment::Environment;
 pub use environment::EnvironmentManager;
 pub use environment::LOCAL_ENVIRONMENT_ID;
 pub use environment::REMOTE_ENVIRONMENT_ID;
+pub use environment_path::EnvironmentPathRef;
 pub use environment_provider::DefaultEnvironmentProvider;
 pub use environment_provider::EnvironmentProvider;
 pub use fs_helper::CODEX_FS_HELPER_ARG1;