[codex] Make justfile recipes Windows-aware (#24983)

## Summary

Make the root `justfile` usable from Windows without maintaining a
separate Windows copy of most recipes.

The repo recipes previously assumed POSIX shell behavior for things like
variadic argument forwarding (`"$@"`) and stderr redirection
(`2>/dev/null`). That made common workflows such as `just fmt`, `just
test`, and `just log` unreliable from Windows. This PR introduces a
small cross-platform shell adapter so recipes can stay mostly unified
while still expanding the few shell-specific constructs correctly on
macOS/Linux and Windows.

## What Changed

- Add `scripts/just-shell.py` as the configured `just` shell adapter.
  - On Unix it invokes `sh -cu`.
- On Windows it invokes `pwsh -CommandWithArgs` so arguments containing
spaces are preserved.
- Add portable recipe placeholders:
- `{args}` expands to `"$@"` on Unix and the equivalent PowerShell
forwarded-args expression on Windows.
- `{stderr-null}` expands to the platform-specific stderr suppression
used by `fmt`.
- Convert most variadic one-line recipes to the unified `{args}` form,
including `codex`, `exec`, `file-search`, `app-server-test-client`,
`fix`, `clippy`, `bench`, `mcp-server-run`, `write-app-server-schema`,
and `argument-comment-lint-from-source`.
- Keep genuinely shell-specific recipes split or Unix-only for now,
including recipes backed by `.sh` scripts or recipes whose bodies are
more than simple command forwarding.
- Add a Windows `just install` path that installs PowerShell via
`winget` when `pwsh` is not available, then runs the same basic Rust
setup steps.
- Update the SDK test that validates the root `fmt` recipe so it
recognizes the new portable stderr placeholder.

## Validation

- `just --summary`
- `just --dry-run fmt`
- `just --dry-run bench-smoke`
- `just --dry-run codex foo "bar binky" baz`
- `just --dry-run write-hooks-schema`
- `just --dry-run bazel-lock-update`
- `just --dry-run argument-comment-lint-from-source -- "foo bar"`
- `git diff --check -- justfile scripts/just-shell.py
sdk/python/tests/test_artifact_workflow_and_binaries.py`
- Verified Windows argv preservation through `scripts/just-shell.py`
with arguments containing spaces.
- `uv run --frozen --project sdk/python --extra dev pytest
sdk/python/tests/test_artifact_workflow_and_binaries.py::test_root_fmt_recipe_formats_rust_and_python_sdk`

sdk/python/tests/test_artifact_workflow_and_binaries.py

@@ -81,14 +81,16 @@ def test_root_fmt_recipe_formats_rust_and_python_sdk() -> None:
     fmt_recipe = lines[fmt_index:next_recipe_index]
     actual = {
         "working_directory": lines[0],
-        "previous_attribute": lines[fmt_index - 1],
+        "previous_comment": next(
+            line for line in reversed(lines[:fmt_index]) if line.startswith("#")
+        ),
         "commands": [line.strip() for line in fmt_recipe[1:] if line.strip()],
     }
     expected = {
         "working_directory": 'set working-directory := "codex-rs"',
-        "previous_attribute": "# Format Rust and Python SDK code.",
+        "previous_comment": "# Format Rust and Python SDK code.",
         "commands": [
-            "cargo fmt -- --config imports_granularity=Item 2>/dev/null",
+            "cargo fmt -- --config imports_granularity=Item {stderr-null}",
             "uv run --frozen --project ../sdk/python --extra dev ruff check --fix --fix-only ../sdk/python",
             "uv run --frozen --project ../sdk/python --extra dev ruff format ../sdk/python",
         ],