docs: add implementation plans for Gemini CLI Workspaces

plans/phase-1-workspace-core.md

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+# Phase 1 Sub-plan: The Workspace Core
+
+## 1. Objective
+Establish the foundational execution environment (Container Image) and the initial management service (Hub API).
+
+## 2. Tasks
+
+### Task 1.1: Define and Build Workspace Image
+Create a Dockerfile that provides a complete, persistent development environment for `gemini-cli`.
+- [ ] Create `packages/grid-manager/docker/Dockerfile`.
+- [ ] Include: `node:20-slim`, `git`, `gh`, `rsync`, `tmux`, `shpool`.
+- [ ] Add the pre-built `gemini-cli` binary.
+- [ ] Define `entrypoint.sh` with secret injection and `shpool` daemon startup.
+- [ ] Verify image build locally: `docker build -t gemini-workspace:v1 .`.
+
+### Task 1.2: Workspace Hub API (v1)
+Implement the core API to manage GCE-based workspaces.
+- [ ] Initialize `packages/grid-manager/src/hub-service/`.
+- [ ] Implement Express or Fastify server for `/workspaces` (List, Create, Delete).
+- [ ] Integrate Firestore to track workspace state (owner, instance_id, status).
+- [ ] Integrate `@google-cloud/compute` for GCE instance lifecycle.
+- [ ] Provision a VM with `Container-on-VM` settings pointing to the `gemini-workspace` image.
+
+### Task 1.3: Cloud Run Deployment (v1)
+Prepare the Hub for self-service deployment.
+- [ ] Create `packages/grid-manager/terraform/` for basic Hub provisioning.
+- [ ] Setup IAP/OAuth authentication on the Cloud Run endpoint.
+
+## 3. Verification & Success Criteria
+- **Image:** A container started from the image must have `gemini --version` and `gh --version` available.
+- **API:** A `POST /workspaces` call must result in a new VM appearing in the specified GCP project with the correct container image.
+- **State:** Firestore must correctly reflect the VM's `PROVISIONING` and `READY` status.
+
+## 4. Next Steps
+- Implement Task 1.1: Build the Dockerfile.