docs: rename Gemini CLI to Antigravity CLI

[devangpratap]

Summary

• Renames user-facing "Gemini CLI" references to "Antigravity CLI" in subtitle and badge
• Renames "Antigravity" to "Antigravity CLI" in the compatibility table (adds "CLI" suffix)
• Keeps the "Gemini CLI" row with install.sh gemini intact so existing users on the pre-rebrand CLI can still find install instructions
• Updates the one-line install header to list both "Antigravity CLI" and "Gemini CLI"
• Same changes applied across the main README, all 7 translated READMEs, and 2 internal docs files
• No code changes, no install.sh changes

Open question for maintainer

install.sh currently has two separate install targets with different paths:

• gemini installs to $HOME/.agents/skills
• antigravity installs to $HOME/.gemini/antigravity/skills

This PR only touches docs. Whether to deprecate/alias/remove the gemini target in install.sh is a separate decision. See discussion in #168.

Files changed (10)

File	Changes
README.md	Badge, subtitle, install header, compatibility table
READMEs/README.*.md (7 files)	Same edits per translation
docs/superpowers/plans/...	Homepage install note
docs/superpowers/specs/...	Homepage design spec

Closes #168

[devangpratap]

@Lum1104 Updated the PR — the Gemini CLI row with install.sh gemini is now preserved in all READMEs alongside the renamed Antigravity CLI entry.

Quick question: install.sh has two separate targets that install to different paths:

• antigravity → $HOME/.gemini/antigravity/skills
• gemini → $HOME/.agents/skills

Should the gemini target stay as-is for backward compatibility, get a deprecation notice pointing users to antigravity, or be removed entirely? Happy to follow up with an install.sh PR if you want option 2 or 3.