#9417 docs(cli): add agent skills setup to the CLI overview

Summary

• Add an "Agent skills" section to docs.sim.ai/cli with the install instructions from the simstudioai/skills README
• List the included skills and the sim plugin namespace, and distinguish them from workspace skills managed with sim skills
• Leave out the bunx sim-skills shortcut and the npm marketplace entry since sim-skills isn't published to npm yet

Type of Change

• Documentation

Testing

Docs-only change to a hand-written page; CI runs lint and check:audits (incl. check:cli-docs)

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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