fix(windows-path): preserve PATH order; include core env vars (#5579)

# Preserve PATH precedence & fix Windows MCP env propagation

## Problem & intent

Preserve user PATH precedence and reduce Windows setup friction for MCP
servers by avoiding PATH reordering and ensuring Windows child processes
receive essential env vars.

- Addresses: #4180 #5225 #2945 #3245 #3385 #2892 #3310 #3457 #4370  
- Supersedes: #4182, #3866, #3828 (overlapping/inferior once this
merges)
- Notes: #2626 / #2646 are the original PATH-mutation sources being
corrected.

---

## Before / After

**Before**  
- PATH was **prepended** with an `apply_patch` helper dir (Rust + Node
wrapper), reordering tools and breaking virtualenvs/shims on
macOS/Linux.
- On Windows, MCP servers missed core env vars and often failed to start
without explicit per-server env blocks.

**After**  
- Helper dir is **appended** to PATH (preserves user/tool precedence).  
- Windows MCP child env now includes common core variables and mirrors
`PATH` → `Path`, so typical CLIs/plugins work **without** per-server env
blocks.

---

## Scope of change

### `codex-rs/arg0/src/lib.rs`
- Append temp/helper dir to `PATH` instead of prepending.

### `codex-cli/bin/codex.js`
- Mirror the same append behavior for the Node wrapper.

### `codex-rs/rmcp-client/src/utils.rs`
- Expand Windows `DEFAULT_ENV_VARS` (e.g., `COMSPEC`, `SYSTEMROOT`,
`PROGRAMFILES*`, `APPDATA`, etc.).
- Mirror `PATH` → `Path` for Windows child processes.  
- Small unit test; conditional `mut` + `clippy` cleanup.

---

## Security effects

No broadened privileges. Only environment propagation for well-known
Windows keys on stdio MCP child processes. No sandbox policy changes and
no network additions.

---

## Testing evidence

**Static**  
- `cargo fmt`  
- `cargo clippy -p codex-arg0 -D warnings` → **clean**  
- `cargo clippy -p codex-rmcp-client -D warnings` → **clean**  
- `cargo test -p codex-rmcp-client` → **13 passed**

**Manual**  
- Local verification on Windows PowerShell 5/7 and WSL (no `unused_mut`
warnings on non-Windows targets).

---

## Checklist

- [x] Append (not prepend) helper dir to PATH in Rust and Node wrappers
- [x] Windows MCP child inherits core env vars; `PATH` mirrored to
`Path`
- [x] `cargo fmt` / `clippy` clean across touched crates  
- [x] Unit tests updated/passing where applicable  
- [x] Cross-platform behavior preserved (macOS/Linux PATH precedence
intact)

codex-rs/rmcp-client/src/utils.rs

@@ -171,13 +171,33 @@ pub(crate) const DEFAULT_ENV_VARS: &[&str] = &[
 
 #[cfg(windows)]
 pub(crate) const DEFAULT_ENV_VARS: &[&str] = &[
+    // Core path resolution
     "PATH",
     "PATHEXT",
+    // Shell and system roots
+    "COMSPEC",
+    "SYSTEMROOT",
+    "SYSTEMDRIVE",
+    // User context and profiles
     "USERNAME",
     "USERDOMAIN",
     "USERPROFILE",
+    "HOMEDRIVE",
+    "HOMEPATH",
+    // Program locations
+    "PROGRAMFILES",
+    "PROGRAMFILES(X86)",
+    "PROGRAMW6432",
+    "PROGRAMDATA",
+    // App data and caches
+    "LOCALAPPDATA",
+    "APPDATA",
+    // Temp locations
     "TEMP",
     "TMP",
+    // Common shells/pwsh hints
+    "POWERSHELL",
+    "PWSH",
 ];
 
 #[cfg(test)]