Windows: allow configuring the default session shell via config

Summary

Codex currently defaults to PowerShell for the Windows session shell path. That makes it hard for users who primarily work in Git Bash or another already-supported shell to make that shell the default for Codex sessions.

I prepared a change that adds an explicit Windows config entry:

[windows]
shell_path = "C:\\Program Files\\Git\\bin\\bash.exe"

When windows.shell_path is configured, Codex uses that executable as the default session shell. When it is not configured, the current behavior remains unchanged and Windows still falls back to PowerShell.

Why this approach

I intentionally implemented this as an explicit config option rather than automatic shell detection:

• Windows does not have a single reliable equivalent to Unix $SHELL
• multiple bash.exe variants may exist on PATH (Git Bash, WSL, MSYS2, etc.)
• explicit configuration is easier to reason about and avoids changing behavior for existing users

Implementation notes

The change currently does the following:

• adds windows.shell_path to config parsing
• supports profile.windows.shell_path overriding the top-level value
• validates the configured executable against Codex's existing built-in shell detection/execution logic
• uses the configured shell as the default session shell when no internal user_shell_override is provided
• updates the Windows shell tool descriptions so they refer to the configured default shell rather than hardcoding PowerShell wording
• updates docs and regenerates config.schema.json

This does not try to add arbitrary shell adapter support. The configured path must resolve to a shell executable already supported by Codex's built-in shell handling.

Testing performed

I ran:

• cargo +stable test -p codex-tools
• cargo +stable test -p codex-core config::tests::config_loads_windows_shell_path_from_toml -- --exact --nocapture
• cargo +stable test -p codex-core config::tests::profile_windows_shell_path_overrides_top_level_value -- --exact --nocapture
• cargo +stable test -p codex-core config::tests::invalid_windows_shell_path_fails_config_load -- --exact --nocapture
• cargo +stable test -p codex-core shell::tests::configured_windows_shell_path_uses_bash -- --exact --nocapture
• cargo +stable test -p codex-core shell::tests::invalid_windows_shell_path_is_rejected -- --exact --nocapture
• cargo +stable test -p codex-core shell::tests::finds_powershell -- --exact --nocapture
• cargo +stable test -p codex-core shell::tests::detects_powershell_as_default -- --exact --nocapture

Branch / commit

I could not open a PR directly because the repository currently limits PR creation to collaborators.

The proposed change is available here:

• branch: LXL47/codex:feat/windows-shell-path-config
• commit: 0d9010493

If this direction is acceptable, I can keep the branch available for maintainers to review or cherry-pick from.