Bridge command generation to powershell when on Windows (#2319)

## What? Why? How?
- When running on Windows, codex often tries to invoke bash commands,
which commonly fail (unless WSL is installed)
- Fix: Detect if powershell is available and, if so, route commands to
it
- Also add a shell_name property to environmental context for codex to
default to powershell commands when running in that environment

## Testing
- Tested within WSL and powershell (e.g. get top 5 largest files within
a folder and validated that commands generated were powershell commands)
- Tested within Zsh
- Updated unit tests

---------

Co-authored-by: Eddy Escardo <eddy@openai.com>

codex-rs/Cargo.lock

@@ -737,6 +737,7 @@ dependencies = [
  "tree-sitter-bash",
  "uuid",
  "walkdir",
+ "which",
  "whoami",
  "wildmatch",
  "wiremock",
@@ -5599,6 +5600,18 @@ version = "0.1.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a751b3277700db47d3e574514de2eced5e54dc8a5436a3bf7a0b248b2cee16f3"
 
+[[package]]
+name = "which"
+version = "6.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b4ee928febd44d98f2f459a4a79bd4d928591333a494a10a868418ac1b39cf1f"
+dependencies = [
+ "either",
+ "home",
+ "rustix 0.38.44",
+ "winsafe",
+]
+
 [[package]]
 name = "whoami"
 version = "1.6.0"
@@ -6011,6 +6024,12 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "winsafe"
+version = "0.0.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d135d17ab770252ad95e9a872d365cf3090e3be864a34ab46f48555993efc904"
+
 [[package]]
 name = "wiremock"
 version = "0.6.4"