Add Docker sandbox backend with backend-aware workspace resolution

[Copilot]

This change adds Docker as an additive sandbox backend behind --sandbox docker while keeping worktree as the default. It also decouples workspace lookup/execution from host-only filesystem paths so clone, create, run, attach, prompt, and remove can target either backend consistently.

• Runtime/backend selection

  • Added a global --sandbox flag with worktree as the default and docker as the additive backend.
  • Switched CLI runtime wiring from a hardcoded worktree layer to backend selection at process startup.

• Backend-aware workspace resolution

  • Introduced WorkspaceRegistryService and WorkspaceHandle as the lookup boundary for workspace selection and execution.
  • Updated run / attach / prompt / list flows to resolve a workspace handle first instead of deriving everything from host cwd strings.
  • Kept the existing filesystem service for host checkout management, but stopped using it as the only source of truth for workspace discovery.

• Docker sandbox backend

  • Added a Docker runtime, sandbox adapter, filesystem adapter, and registry implementation.
  • Docker workspaces are identified from container labels/names rather than host directories.
  • Added lazy image bootstrap from a Dockerfile under ~/.local/share/skipper/docker.
  • The image starts from Ubuntu and installs Node, Bun, GitHub CLI, and OpenCode.

• Repo snapshot + branch creation behavior

  • Docker workspace creation copies a refreshed default-branch snapshot from the host checkout into the container without mutating the user’s checkout.
  • The copied snapshot includes .git metadata, then branch workspaces create/check out the target branch inside the container.
  • Main gets its own container; each branch workspace gets its own long-lived container.

• Command/prompt parity

  • workspace run now executes through backend-specific workspace handles.
  • workspace attach uses backend-aware attach behavior; Docker attaches via docker exec -it <container> bash.
  • workspace prompt is container-aware and runs OpenCode against the container workspace rather than assuming a host path.
  • workspace remove routes cleanup through the selected backend, including Docker container removal.
  • Docker detach is treated as a no-op.

• Docs and coverage

  • Updated the README to document Docker as an optional backend and show --sandbox docker usage.
  • Added focused tests around sandbox flag parsing, workspace registry behavior, and the updated workspace-handle-based prompt/list flows.

Example usage:

# Clone host checkout + create main container
sk clone git@github.com:owner/repo.git --sandbox docker

# Create a branch workspace container from the default-branch snapshot
sk workspace create --repository repo --branch feat/x --sandbox docker

# Run inside the container workspace
sk workspace run --repository repo --branch feat/x --command 'pwd' --sandbox docker

# Prompt OpenCode inside the container workspace
sk workspace prompt --repository repo --branch feat/x "Explain this codebase" --sandbox docker