Docs mismatches: sandbox link, Homebrew install command, and Windows support wording

Tracking a few docs inconsistencies between the public repo and current Codex docs/behavior.

Confirmed items:

1. docs/sandbox.md links to https://developers.openai.com/codex/security, which is now the Codex Security product page rather than the sandbox/approvals docs. The intended destination appears to be https://developers.openai.com/codex/agent-approvals-security.

1. The repo README(s) say brew install --cask codex, while the official quickstart says brew install codex. If the repo is the source of truth, the official docs should match or the repo should clarify why the cask path is preferred.

1. docs/install.md still says Windows 11 is supported only via WSL2, but the repo and current docs both reference native Windows sandbox support. The install doc appears stale.

I’m preparing a small PR for the repo-owned fixes (1 and 3). I’m leaving the official-docs-side Homebrew mismatch here for visibility because that change may need to happen outside this repo unless the repo wording is intentionally being updated instead.