Make all shortcut commands installable via pip (cross-shell, cross-platform)

[luckeyfaraday]

Problem

Before this PR, only codex-shim was registered as a real pip entry point. The other shortcuts — codex-app, codex-model, codex-openrouter, codex-minimax — existed only as bash scripts in bin/. That meant:

• On Windows, they could not be invoked from PowerShell or cmd, only from Git Bash / WSL.
• On POSIX, they required manual ln -s into ~/.local/bin.

Change

• Add four tiny shim functions in codex_shim/cli.py that re-enter main() with the right argv prefix.
• Register them under [project.scripts] in pyproject.toml.
• Update the README install section to recommend pip install -e .. The POSIX ln -s route is kept as an alternative.

The bash scripts in bin/ are untouched so existing setups keep working.

Verification

Installed locally with pip install -e . on Windows 11 / Python 3.11. All five commands resolve to .exe wrappers in the Python install's Scripts\ dir:

codex-shim.exe
codex-app.exe
codex-model.exe
codex-openrouter.exe
codex-minimax.exe

Confirmed codex-shim provider list and codex-shim --help run identically from PowerShell and Git Bash, from arbitrary directories.

Test plan

• python -m pytest -q (22 passed)
• codex-shim provider list from PowerShell in ~
• codex-shim provider list from Git Bash in ~
• On a Linux/macOS box: pip install -e . then run codex-openrouter from a fresh shell

[luckeyfaraday]

Updated this PR with the Windows provider shortcut fix:

• keep bin/codex-* scripts LF via .gitattributes to avoid Bash pipefail CRLF failures
• make Windows codex-openrouter/codex-minimax entry points delegate to the Bash helpers so interactive Codex keeps a usable TTY
• restore WSL venv/PATH handling and longer shim startup wait
• added regression coverage for Windows provider entry delegation

Verification: python -m pytest (23 passed)