chore(dev): one-command dev start script (onboarding-first)

[Shakalito]

Provide a single command to bootstrap the full dev environment.

AC:

• Runs from repo root

• Verifies .env; creates from template if missing

• Starts Docker services, backend, frontend

• Clear console output with ports and status

Deps

• chore(dev): implement robust absolute path resolution for .env and project root #52