feat(gemini): add installation script and docs for Gemini CLI / Antigravity

[cwest]

This PR adds first-class support for Google's Gemini CLI and Antigravity tools. It includes installation instructions and a script to inject the necessary context into the Gemini environment, ensuring feature parity with other integrations like Codex and OpenCode.

Summary by CodeRabbit

• New Features

  • Added Gemini CLI / Antigravity integration support with installation and setup documentation.

• Bug Fixes

  • Fixed SessionStart hook execution to ensure context availability on first message.

• Documentation

  • Updated README with Gemini CLI integration details and expanded Skills Library information.
  • Added installation and management guide for the Superpowers feature.

• Chores

  • Version bumped to 4.3.0.

[coderabbitai]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

This PR releases version 4.3.0 of the Superpowers plugin by updating version numbers in manifest files, introducing Gemini CLI integration with installation documentation and setup scripts, and updating README with new workflow details and skills library entries.

Changes

Cohort / File(s)	Summary
Plugin Version Bumps .claude-plugin/marketplace.json, .claude-plugin/plugin.json	Version incremented from 4.2.0 to 4.3.0 in both public and internal plugin manifests.
Gemini CLI Integration .gemini/INSTALL.md, .gemini/install.sh	New documentation and automated setup script for installing Superpowers in Gemini CLI environments, including symlink creation and context block injection into GEMINI.md.
Documentation Updates README.md, RELEASE-NOTES.md	README expanded with Gemini CLI section, new skills library entries, and installation details; release notes document v4.3.0 changes including brainstorming workflow enforcement and synchronous SessionStart hook execution.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• fix(opencode): standardize on plugins/ directory, fix symlink docs #349: Modifies the same .claude-plugin/plugin.json version field during a version bump.

Poem

🐰 Whiskers twitch with glee—
Version 4.3's here, you see!
Gemini CLI joins the spree,
Skills and docs for all to be. ✨

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[cwest]

Closing per user request to rebase and update.