docs(offload): implement maintainer onboarding guide and link from contributing

MAINTAINER_ONBOARDING.md

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+# Maintainer Onboarding: High-Performance Offload System 🚀
+
+Welcome to the Gemini CLI maintainer team! This guide will help you set up your
+remote development environment, which offloads heavy tasks (reviews, fixes,
+preflight) to a dedicated GCP worker.
+
+## Prerequisites
+
+1. **Google Cloud Access**: Ensure you have access to the
+   `gemini-cli-team-quota` project.
+2. **GCloud CLI**: Authenticated locally (`gcloud auth login`).
+3. **GitHub CLI**: Authenticated locally (`gh auth login`).
+4. **iTerm2**: (Optional) For automated window popping on macOS.
+
+## Setup Workflow
+
+### 1. Fork & Clone
+
+Start by forking the repository and cloning it to your local machine.
+
+```bash
+gh repo fork google-gemini/gemini-cli --clone
+cd gemini-cli
+```
+
+### 2. Run the Offload Setup
+
+This interactive script will handle all the complex orchestration setup:
+
+- Configures your GCP project and compute zone.
+- Sets up a **Fast-Path SSH Alias** (`gcli-worker`) in `~/.ssh/config`.
+- Creates/Identifies your **Security Fork** for autonomous work.
+- Performs a **One-Shot Authentication** for Gemini and GitHub.
+- Pre-clones the repository to your remote worker.
+
+```bash
+npm run offload:setup
+```
+
+### 3. Provision Your Worker
+
+Once setup is configured, spin up your dedicated, high-performance VM:
+
+```bash
+npm run offload:fleet provision
+```
+
+## Daily Workflow
+
+### Offloading a PR Review
+
+To perform a deep behavioral review of a PR on your remote worker:
+
+```bash
+npm run offload <PR_NUMBER> review
+```
+
+_A new iTerm2 window will pop up, instantly connected to your worker, running
+the `review-pr` skill._
+
+### Monitoring Your Jobs
+
+View the real-time status of all your in-flight remote jobs:
+
+```bash
+npm run offload:status
+```
+
+### Stopping Your Worker
+
+To save costs, shut down your worker when you're done for the day. The
+orchestrator will automatically wake it up when you run a new task.
+
+```bash
+npm run offload:fleet stop
+```
+
+## Security Model
+
+- **Isolation**: Each maintainer has their own dedicated VM
+  (`gcli-offload-<user>`).
+- **Permissions**: The agent uses a scoped token that is read-only to the main
+  repo and read/write only to your personal fork.
+- **OS Login**: Access is managed via your Google corporate identity.