refactor: normalize unix module layout for exec-server and shell-escalation (#12556)

## Why
Shell execution refactoring in `exec-server` had become split between
duplicated code paths, which blocked a clean introduction of the new
reusable shell escalation flow. This commit creates a dedicated
foundation crate so later shell tooling changes can share one
implementation.

## What changed
- Added the `codex-shell-escalation` crate and moved the core escalation
pieces (`mcp` protocol/socket/session flow, policy glue) that were
previously in `exec-server` into it.
- Normalized `exec-server` Unix structure under a dedicated `unix`
module layout and kept non-Unix builds narrow.
- Wired crate/build metadata so `shell-escalation` is a first-class
workspace dependency for follow-on integration work.

## Verification
- Built and linted the stack at this commit point with `just clippy`.

[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/12556).
* #12584
* #12583
* __->__ #12556

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

@@ -1,11 +1,5 @@
 #[cfg(unix)]
-mod posix;
+mod unix;
 
 #[cfg(unix)]
-pub use posix::main_execve_wrapper;
-
-#[cfg(unix)]
-pub use posix::main_mcp_server;
-
-#[cfg(unix)]
-pub use posix::ExecResult;
+pub use unix::*;