Update deprecated Gemini CLI docs with Antigravity CLI

[siddqamar]

Antigravity CLI

Summary

• Rename the Gemini CLI agent page to Antigravity CLI
• Update the page description, overview, and command table
• Refresh references across related docs pages
• Update the GitHub repository link
• Remove outdated Gemini CLI wording from this entry

Why

• Gemini CLI is now superseded in this docs path
• The directory should reflect the current agent name and positioning
• Related references need to stay consistent across the site

Verification

• Tested locally

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