Setup Devcontainer

Generates .devcontainer/ from the templates in templates/: one Shared Container — a single devcontainer.json and Dockerfile — that every selected AI CLI installs into via its own independent skill, run separately after this one. See CONTEXT.md for this skill's vocabulary (Shared Container, Local Checkout, Shared Checkout, Scaffold, CLI Skill, Own-Block Contract, Project-Owned Block, Capability Seam, Network Manifest, Project Mounts).

This skill owns the container definition (devcontainer.json, Dockerfile) and the shared parts of post-create.sh/post-start.sh/README.md entirely — no CLI skill ever edits them directly, except for one narrow, explicit exception (the Capability Seam, runArgs, used by setup-codex-devcontainer).

1. Resolve the workspace root, then detect the target repo