Add Swift 6.2 support

[lkeude96]

Summary

I'm working on a Swift package that targets Swift 6.2, so I added first-class support for that toolchain. Swift 6.2 became an officially supported release on September 15, 2025, so the image now installs it by default alongside earlier versions. Details: https://www.swift.org/blog/swift-6.2-released/

Changes

• install Swift 6.2 on amd64 builds while keeping 6.1 and 5.10.1 available via mise
• update setup scripts and documentation to list Swift 6.2 as a supported selectable version
• refresh SBOM metadata for the Swift package version
• bump the pinned libpq-dev dependency to 16.10-* so apt still resolves

Testing

• docker build --platform linux/amd64 -t codex-universal:swift-6.2 . --progress=plain