Update links for the d2lang organization

[alixander]

What changed

• Point current repository, release, badge, documentation, and D2.js metadata links at the new d2lang organization.
• Change the install script release source to d2lang/d2 and regenerate install.sh from its source template.
• Remove the issue workflow that still added new issues to Terrastruct Project 34.
• Preserve compatibility identifiers including oss.terrastruct.com/d2, @terrastruct/d2, terrastruct/d2 on Docker Hub, TALA links, and the shared terrastruct/ci submodule.

Why

GitHub redirects make the old repository URLs work today, but active install and documentation paths should not depend on the old organization namespace.

Validation

• git diff --check
• sh -n ci/release/_install.sh install.sh
• jq empty d2js/js/package.json
• Regenerated installer is deterministic
• Installer dry run exits successfully and resolves releases from github.com/d2lang/d2
• Updated repository and release targets return HTTP 200

Go tests were not run locally because Go is not installed in this environment.