docs: update README.md to mention curl-based installer

[bolinfest]

Now that users can install via curl (or irm), we should tell them about it so they no longer need to use npm!

Note that on one Windows machine I tested on, when I ran:

irm https://chatgpt.com/codex/install.ps1 | iex

I got this error:

iex : The property 'OSArchitecture' cannot be found on this object. Verify that the property exists.
At line:1 char:45
+ irm https://chatgpt.com/codex/install.ps1 | iex
+                                             ~~~
    + CategoryInfo          : NotSpecified: (:) [Invoke-Expression], PropertyNotFoundException
    + FullyQualifiedErrorId : PropertyNotFoundStrict,Microsoft.PowerShell.Commands.InvokeExpressionCommand

so we'll recommend the following that works from both cmd.exe and PowerShell:

powershell -ExecutionPolicy ByPass -c "irm https://chatgpt.com/codex/install.ps1 | iex"

This PR makes a slight update to codex-rs/tui/src/update_action.rs to match.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid requiring pwsh in Windows install command

This line says to run the command in PowerShell but then pipes the script to pwsh, which is only available when PowerShell 7 is installed; on many Windows setups that only have Windows PowerShell (powershell.exe), the command fails with pwsh not found and the installer cannot run. Because this README section is the primary quickstart path, the instruction should execute in the current shell (for example via iex) or explicitly call powershell instead of requiring a separate executable.

Useful? React with 👍 / 👎.

[bolinfest]

Can you give me a concrete alternative? The PR body describes the error I ran into with iex.