base: add pi-devbox-version command + startup banner

Wraps /etc/pi-devbox/build-manifest.json (already written at docker-build
time in Dockerfile.variant) into a human-readable summary instead of
requiring users to know the manifest path and pipe it through jq
themselves.

- rootfs/usr/local/bin/pi-devbox-version: human (default) / --json /
  --quiet output modes. Also flags live drift — compares the baked
  pi_version against the actually-running `pi --version` and warns
  on mismatch rather than trusting the manifest blindly (same
  ground-truth philosophy as the manifest generation itself). Exits 1
  with a short stderr notice on images built before the manifest
  existed, instead of failing silently.
- entrypoint-user.sh: calls it as the very first line. CMD is
  `bash -l` with tty:true in compose, so this banner lands directly
  above the first prompt on container start — no separate motd/bashrc
  hook needed (deliberately not wired into .bash_aliases, which would
  reprint on every docker exec -it).
- Dockerfile.base: COPY + chmod, same pattern as dot-watch/studio-expose.
- scripts/smoke-test.sh: 4 new checks (binary present+executable, human
  output has release tag, --json round-trips the manifest, --quiet is
  a single line).
- README.md / AGENTS.md / CHANGELOG.md updated.

Dockerfile.base

@@ -604,11 +604,13 @@ COPY rootfs/usr/local/lib/pi-devbox/ /usr/local/lib/pi-devbox/
 COPY rootfs/usr/local/share/pi-devbox/ /usr/local/share/pi-devbox/
 COPY rootfs/usr/local/bin/studio-expose /usr/local/bin/studio-expose
 COPY rootfs/usr/local/bin/dot-watch /usr/local/bin/dot-watch
+COPY rootfs/usr/local/bin/pi-devbox-version /usr/local/bin/pi-devbox-version
 COPY entrypoint.sh /usr/local/bin/entrypoint.sh
 COPY entrypoint-user.sh /usr/local/bin/entrypoint-user.sh
 RUN chmod +x /usr/local/bin/entrypoint.sh /usr/local/bin/entrypoint-user.sh \
     /usr/local/bin/studio-expose \
     /usr/local/bin/dot-watch \
+    /usr/local/bin/pi-devbox-version \
     /usr/local/lib/pi-devbox/*.sh 2>/dev/null || true
 
 # Start as root — entrypoint adjusts UID/GID then drops to developer