Restructure nvim sandbox into "devcube" IDE with zellij + workmux

[binarypie]

Turn the single-purpose nvim-dev sandbox into a containerized IDE that can run multiple AI agents in parallel, each in its own git worktree, inside one container. Adds zellij (multiplexer) + workmux (git-worktree orchestration) and a baked fish + starship shell, behind a single devc command.

• Move the container project dot_files/nvim -> dot_files/devcube (Containerfile, entrypoint, Justfile, README, wrapper). dot_files/nvim now holds only the nvim config dotfile.
• Build context moves up to dot_files/ so fish/starship/zellij/workmux/nvim configs (top-level dotfiles) can be baked in; add dot_files/.dockerignore.
• New wrapper devcube.sh dispatches on its first arg (devc -> workmux, devc nvim|claude|codex|agy|zellij), replacing the per-tool symlinks that shadowed host installs. Orchestrators get a per-project worktree volume mounted at /worktrees (path-derived name, persists across restarts, isolated per project) with workmux state redirected there via XDG_STATE_HOME.
• New baked configs: zellij Tokyo Night Moon theme + workmux layout, workmux config (worktree_dir: /worktrees).
• Full rename: image -> devcube, workflow -> build-devcube.yml, ujust recipes -> devcube-, just file -> 61-devcube.just, env -> DEVCUBE_, sentinel -> DEVCUBE, state volume -> hypercube-devcube-home.
• Update nvim-launcher/desktop to open devc nvim, plus docs and the build test reference.

Claude-Session: https://claude.ai/code/session_01K44UxeGUD7Peow4F4ipmXz