Add Docker sandbox backend with runtime repo copy

[blntrsz]

Goal

Add a Docker sandbox backend with parity to the current worktree sandbox.

Scope

• Add Docker as an additive backend behind --sandbox docker
• Keep worktree as default backend
• Preserve current main UX while resolving the actual GitHub default branch internally
• Make Docker support clone, workspace create, workspace run, workspace attach, workspace prompt, and workspace remove

Product decisions

• Source of truth for repo contents is the host checkout
• Before copying, refresh against remote default branch without mutating the user's host checkout
• Main gets its own container
• Each branch workspace gets its own long-lived container
• Branch creation copies the default-branch repo, then creates/checks out the target branch inside the container
• Repo copy includes full git metadata, not just files
• Docker workspace state is a snapshot at init time; no auto-resync after creation
• workspace prompt runs inside the container
• workspace attach uses docker exec -it <container> bash
• Docker detach is a no-op
• Docker labels/names are the source of truth for workspace lookup/listing
• Build the image lazily on first Docker use from a Dockerfile under ~/.local/share/skipper
• Base image starts from Ubuntu and installs Node, Bun, and OpenCode
• Mount host auth/config read-only for OpenCode, git, GitHub CLI, and ssh
• Keep current repo-name identity at the CLI surface for now

Architecture

Runtime selection

• Add a global --sandbox flag
• Default to worktree
• Use DI/runtime wiring to provide the selected backend instead of hardcoding WorkTreeSandboxServiceLayer

Core interfaces

• Keep SandboxService as the main lifecycle boundary if possible
• Add backend-aware workspace registry / locator / handle services so run/prompt/list flows do not depend on raw host cwd strings
• Keep the existing filesystem service for host checkout management, but stop treating it as the only workspace lookup source

Docker backend behavior

• Ensure a clean default-branch source snapshot from the host-side repo data
• Create a main container for repo-level clone flow
• Create one container per branch workspace
• Copy the repo into the container at runtime
• For branch workspaces, create/check out the branch inside the container after copy
• Execute commands in the container workspace
• Run OpenCode inside the container for prompt flow
• Remove container resources on workspace destroy

Workspace lookup

• Worktree backend keeps current filesystem-based lookup
• Docker backend lists/locates workspaces from Docker labels and container names
• Shared workspace selection should route through a backend-aware registry layer

Implementation phases

1. Add backend selection plumbing for --sandbox with worktree as default
2. Add backend-aware workspace registry / locator / handle abstractions
3. Add Docker image bootstrap and lazy image build flow
4. Add Docker sandbox adapter for init, execute, attach, detach, and destroy
5. Make prompt flow container-aware so OpenCode runs inside Docker
6. Update listing/selection/docs/tests and verify both backends

Likely files

• packages/core/src/runtime/local-work-tree.runtime.ts
• packages/core/src/workspace/port/sandbox.service.ts
• packages/core/src/workspace/use-case/init-workspace.use-case.ts
• packages/core/src/workspace/use-case/run-command-in-workspace.use-case.ts
• packages/core/src/workspace/use-case/prompt-workspace.use-case.ts
• packages/core/src/workspace/use-case/list-main-project.use-case.ts
• packages/core/src/workspace/use-case/list-branch-project.use-case.ts
• packages/cli/src/common/global-flags.ts
• packages/cli/src/index.ts
• packages/cli/src/command/clone.command.ts
• packages/cli/src/command/workspace/workspace.common.ts
• new Docker runtime / adapter / registry helpers under packages/core/src/runtime/ and packages/core/src/workspace/adapter/

Verification

• sk clone git@github.com:owner/repo.git --sandbox docker creates host checkout + main container
• sk workspace create --repository repo --branch feat/x --sandbox docker creates a branch container from default-branch source
• sk workspace run --repository repo --branch feat/x --command 'pwd' --sandbox docker runs inside the right container workspace
• sk workspace attach --repository repo --branch feat/x --sandbox docker opens an interactive shell in the container
• sk workspace prompt --repository repo --branch feat/x 'Explain this codebase' --sandbox docker runs OpenCode inside the container
• sk workspace remove --repository repo --branch feat/x --sandbox docker cleans up container resources
• worktree backend still works unchanged by default
• run bun test and bun run check:fix