Add Gemini CLI plugin

[priyanshuharshbodhi1]

Feature request

Add support for Gemini CLI as a built-in AO agent plugin.

Why

AO already supports multiple coding agents like Claude Code, Codex, Aider, and OpenCode. Gemini CLI would be useful as another first-class option so users can configure:

defaults:
  agent: gemini

Expected behavior

• Gemini CLI can be selected as an AO agent
• AO can spawn Gemini-backed sessions
• Follow-up actions like sending more instructions to a running session work normally
• Restore/status/session handling works the same way as other built-in agents

Use case

Some users already use Gemini CLI locally and want to use it through AO instead of switching to a different coding agent.