Reorganize AI documentation for Antigravity

[lamek]

Reorganize the Flutter AI and development tooling documentation to support the transition from Gemini CLI and Code Assist to the new Antigravity 2.0 toolset, Antigravity IDE, and Antigravity CLI.

Summary of Changes:

1. Antigravity 2.0 & IDE Separation:

   • Relocated the tool documentation page from /tools/antigravity into the unified AI section at /ai/antigravity.
   • Updated content to distinguish between the Antigravity 2.0 agentic suite and the Antigravity IDE editor panel.
   • Updates references across hot-reload.md, faq.md, and create-new-app.md to point to the new path.
   • Closes Document Antigravity 2.0 and Antigravity IDE as separate tools #13405

2. New Antigravity CLI Documentation:

   • Created the /ai/antigravity-cli guide documenting the executable command agy, installation on macOS/Linux/Windows, onboarding, and configuring folder rules (AGENTS.md and .agents/skills/).
   • Closes Document support for Antigravity CLI #13404

3. Superseded Gemini CLI References & Redirects:

   • Designated Gemini CLI and extension references as legacy for individual developers.
   • Refactored coding-assistants.md and create-with-ai.md to align with the new CLI.
   • Configured 301 redirects in firebase.json (/tools/antigravity -> /ai/antigravity and /ai/gemini-cli-extension -> /ai/antigravity-cli).
   • Closes Evaluate mentions of Gemini CLI and Code Assist for updates to Antigravity #13403

Sidenav & Navigation Updates:

• Updated ai.yml navbar to include Antigravity IDE and Antigravity CLI under AI coding assistants.
• Removed the old /tools/antigravity listing from default.yml sidebar.

[gemini-code-assist]

Code Review

This pull request migrates documentation from the legacy Gemini CLI and its Flutter extension to the new Antigravity CLI and Antigravity IDE suite, moving the relevant pages under the /ai/ directory and updating navigation and redirects accordingly. The review feedback suggests minor grammatical and terminology corrections in the new antigravity-cli.md file, specifically fixing a pronoun agreement issue and clarifying a reference to the global configuration directory.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 0d3129d):

https://flutter-docs-prod--docs-pr13470-antigravity-docs-reorg-o5239u6o.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 0d3129d):

https://flutter-dev-230821--www-pr13470-antigravity-docs-reorg-9y492fod.web.app