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/shell-escalation/src/unix/escalate_protocol.rs

@@ -0,0 +1,54 @@
+use std::collections::HashMap;
+use std::os::fd::RawFd;
+use std::path::PathBuf;
+
+use serde::Deserialize;
+use serde::Serialize;
+
+/// 'exec-server escalate' reads this to find the inherited FD for the escalate socket.
+pub const ESCALATE_SOCKET_ENV_VAR: &str = "CODEX_ESCALATE_SOCKET";
+
+/// Patched shells use this to wrap exec() calls.
+pub const EXEC_WRAPPER_ENV_VAR: &str = "EXEC_WRAPPER";
+
+/// Compatibility alias for older patched bash builds.
+pub const LEGACY_BASH_EXEC_WRAPPER_ENV_VAR: &str = "BASH_EXEC_WRAPPER";
+
+/// The client sends this to the server to request an exec() call.
+#[derive(Clone, Serialize, Deserialize, Debug, PartialEq, Eq)]
+pub struct EscalateRequest {
+    /// The absolute path to the executable to run, i.e. the first arg to exec.
+    pub file: PathBuf,
+    /// The argv, including the program name (argv[0]).
+    pub argv: Vec<String>,
+    pub workdir: PathBuf,
+    pub env: HashMap<String, String>,
+}
+
+/// The server sends this to the client to respond to an exec() request.
+#[derive(Clone, Serialize, Deserialize, Debug, PartialEq, Eq)]
+pub struct EscalateResponse {
+    pub action: EscalateAction,
+}
+
+#[derive(Clone, Serialize, Deserialize, Debug, PartialEq, Eq)]
+pub enum EscalateAction {
+    /// The command should be run directly by the client.
+    Run,
+    /// The command should be escalated to the server for execution.
+    Escalate,
+    /// The command should not be executed.
+    Deny { reason: Option<String> },
+}
+
+/// The client sends this to the server to forward its open FDs.
+#[derive(Clone, Serialize, Deserialize, Debug)]
+pub struct SuperExecMessage {
+    pub fds: Vec<RawFd>,
+}
+
+/// The server responds when the exec()'d command has exited.
+#[derive(Clone, Serialize, Deserialize, Debug)]
+pub struct SuperExecResult {
+    pub exit_code: i32,
+}