Fix README skills path and repo URLs; add .gitignore

[CloudAI-X]

Summary

Fixes documentation bugs introduced in the README and prevents .DS_Store from recurring — findings from a code review of the 5 most recent commits authored by @CloudAI-X (they had no authored PRs to review).

README.md

• Skills live in skills/, not .claude/skills/. Corrected the directory in the Installation, How It Works, and Contributing sections so the instructions actually match the repo layout.
• Installation clone/submodule commands pointed at github.com/pinkforest/threejs-playground.git; changed to github.com/CloudAI-X/threejs-skills.git.

.gitignore (new)

• Commits e2b7cff and 260ead4 deleted .DS_Store files but no ignore rule existed, so they would be recommitted. Adds .DS_Store / **/.DS_Store.

Not included (flagged in review, left as-is): the README still refers to "Claude Code" throughout while acknowledgments were trimmed, and the "audited against r160+" claim is drifting (PR #6 bumps to r183) — happy to follow up if wanted.

Link to Devin session: https://app.devin.ai/sessions/ff96459e5b524382a21d7979f7b68790

[devin-ai-integration]

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