umbra-core v0.4.0

Added — CLI developer experience

• umbra init scaffolds a conservative starter .umbra/admission.yaml (loads
  cleanly through the real contract loader; refuses to overwrite without --force),
  so a new user is one command from a governed change.
• umbra completion <bash|zsh|fish> prints a shell completion script for the
  subcommands (dependency-free; eval "$(umbra completion zsh)").
• install.sh — a curl … | sh one-line installer (uv → pipx → pip, isolated
  and fail-closed) and a Homebrew tap (brew install bkd-dotcom/umbra/umbra).
• Docs site: a Capabilities & Proof page (capability graph, plan binding,
  dual/masked verifier, G1/G2/G3 gates, extension admission) and a refreshed
  quickstart.