v2.7.3 — version banner on install

Both install.sh and cursor-install.sh now show a banner with the version
number at the top of the run, sourced from a new VERSION file at the
repo root.

cursor-install.sh also reads VERSION into the haze-config.json scaffold,
so future version bumps are one-line VERSION edits — no other code
changes needed for version display.

Canonical hashes:
  install.sh sha256:        b68a9d762b96635bc387acccf451f2af56765712a80366cec7dcf39047e2b2e2
  cursor-install.sh sha256: b663cf825ad6cf12dcb6b82896a9c5ac62dd53e11372badad0fe4152e3b16ea5