[codex] Update xlings ecosystem references

[Sunrisepeak]

Summary

• Update GitHub Actions and devcontainer bootstrap to use openxlings/xlings and pin xlings to v0.4.14.
• Replace stale d2learn/xlings repository links and legacy install-script URLs with canonical openxlings/xlings links.
• Fix old xlings checker wording to d2x checker and replace the broken d2learn.org/docs/xlings AI tips links with current xlings d2x documentation pages.

Notes

I checked mdbook@0.5.2 because it is the current xlings index latest, but it rejects the existing book.toml author field. This PR keeps the existing mdbook@0.4.43 pin to avoid mixing a compatibility migration into the xlings reference update.

Validation

• rg -n "github\.com/d2learn/xlings|raw\.githubusercontent\.com/d2learn/xlings|d2learn\.org/xlings-install|d2learn\.org/docs/xlings|xlings checker|XLINGS_VERSION: \"0\.4\.3\"|v0\.4\.2" --glob '!**/.git/**' returned no matches.
• git diff --check
• bash -n .devcontainer/postCreate.sh book/build.sh tools/update_upstream.sh
• curl -fsI checked the new xlings release asset, quick install scripts, and d2x documentation URLs.
• mdbook v0.4.43 && ./book/build.sh completed; existing mdbook-foo preprocessor warnings remain.
• xmake f -y
• Built and ran all 47 *-ref targets through xmake d2x-buildtools.
• Filename parity check reports the existing informational warning for missing solutions/hello-mcpp.cpp.