Update Obsidian plugin links for d2lang

[alixander]

What changed

• Update active D2 and plugin source, issue, badge, theme, and generated-bundle links for the d2lang organization.
• Point the shared CI submodule at d2lang/ci so new clones no longer depend on the Terrastruct repository path.
• Add standard homepage, bugs, and repository package metadata.
• Preserve the Obsidian plugin ID and author metadata.

Validation

• Frozen dependency install
• TypeScript no-emit check
• Production esbuild
• JSON and link checks
• Fresh CI submodule initialization from https://github.com/d2lang/ci.git
• git diff --check

The repository's existing actions/cache@v2 usage causes the GitHub Actions ci job to stop during setup; that pre-existing workflow compatibility issue is outside this link-migration PR.