Add Google Gemini CLI agent integration

[Copilot]

Adds a Gemini CLI-backed Rig engine with persistent multi-turn sessions.

Changes

• Engine

  • Runs an installed, authenticated gemini CLI in headless JSON mode.
  • Preserves conversations with --session-id and --resume.
  • Supports custom command, working directory, arguments, environment, and approval mode.

• Lifecycle

  • Propagates cancellation to the CLI process.
  • Rejects pre-aborted and concurrent turns.
  • Waits for process exit, escalating to SIGKILL when necessary.

• API

  • Exports geminiEngine from rig/engines/gemini.
  • Rejects Rig tools because the CLI does not expose custom tool registration.

import { configureAgent } from "rig";
import { geminiEngine } from "rig/engines/gemini";

configureAgent(geminiEngine({ approvalMode: "plan" }));