feat(build): implement multi-stage maintainer parent image and dual-tag cloud build

.gemini/skills/offload/NEXT_MISSION.md

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+# Mission: GCE Container-First Refactor 🚀
+
+## Current State
+- **Architecture**: Persistent GCE VM (`gcli-offload-mattkorwel`) with Fast-Path SSH (`gcli-worker`).
+- **Logic**: Decoupled scripts in `~/.offload/scripts`, using Git Worktrees for concurrency.
+- **Auth**: Scoped GitHub PATs mirrored via setup.
+
+## The Goal (Container-OS Transition)
+Shift from a "Manual VM" to an "Invisible VM" (Container-Optimized OS) that runs our Sandbox Docker image directly.
+
+## Planned Changes
+1. **Multi-Stage Dockerfile**: 
+   - Optimize `Dockerfile.gemini-maintainer` to include `tsx`, `vitest`, `gh`, and a pre-warmed repository.
+   - Base it on the existing `google-gemini-cli-sandbox`.
+2. **Setup Script (`setup.ts`)**:
+   - Refactor `provision` to use `gcloud compute instances create-with-container`.
+   - Configure the VM to launch the container as the primary entrypoint.
+3. **Orchestrator (`orchestrator.ts`)**:
+   - Update SSH logic to include the `--container` flag.
+   - Ensure `rsync` still maps to the persistent home directory that is mounted into the container.
+
+## How to Resume
+1. Load the checkpoint: `/checkpoint load offload-container-refactor` (if available).
+2. Tell Gemini: *"Read .gemini/skills/offload/NEXT_MISSION.md and start Phase 1: The Multi-Stage Dockerfile."*