Greet new terminals in the dev container, and restart the backend

[Decipher]

Stacked on #31.

Summary

Open a terminal in the dev container and nothing greets you. This ports the
mechanism from the druxt.js contributor dev container (gitlab.local MR !47) so
it does.

Why nothing printed

The splash and the next-steps block do exist: scripts/postinstall.mjs prints
both. They just never reach a person.

postinstall runs from npm install, which this container runs inside
postCreateCommand, so its output goes to the container creation log. A
postAttachCommand would be no better: its output goes to whatever launched the
container, and DevPod prints that to its own CLI log, outside the container
entirely.

The shell's own rc file is the only place that reliably reaches someone who has
just opened a terminal.

Changes

• .devcontainer/WELCOME.txt, the greeting: what is running, what ports are
  forwarded, and the handful of commands worth knowing.
• post-create.sh appends a hook to both ~/.bashrc and ~/.zshrc. Both,
  because the base image ships bash and zsh and which one a terminal opens
  depends on the client. On every interactive shell rather than once, because a
  one-shot marker gets spent by whichever shell the editor opens first, which is
  rarely the one the user is looking at. It also prints the guide at the end of
  post-create, for anyone watching the creation log.
• .devcontainer/post-start.sh and a postStartCommand. The container had
  no start hook at all, so stopping and resuming it left the frontend pointing
  at a backend that was no longer listening. That looks like a broken install
  rather than a stopped process. It skips with a pointer when the site was never
  provisioned, so one broken create does not also fail every start.
• Codespaces opens the guide as a file on first attach.

SPLASH in scripts/lib.mjs stays as it is: setup.mjs and postinstall.mjs
both use it, and they are still the right place for it when someone runs setup
by hand.

Test plan

• devpod up github.com/druxt/quickstart-druxt-serverless@feature/devcontainer-welcome --id qds-pr32 --ide openvscode
• Opening a terminal prints the guide
• Opening a second terminal prints it again
• Stopping and resuming the container leaves the backend serving on 8888

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