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.

CHANGELOG.md

@@ -13,6 +13,22 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
 
 ## Unreleased
 
+### Added
+
+- **`pi-devbox-version` command.** Wraps `/etc/pi-devbox/build-manifest.json`
+  into a human-readable summary (release tag, build date, source revision,
+  baked `pi_version`, and short SHAs for every `/opt` component) instead of
+  requiring users to know the manifest path and pipe it through `jq`
+  themselves. Also flags **live drift** — if `pi --version` no longer matches
+  what was baked at build time, the `pi:` line calls that out rather than
+  silently trusting the manifest. `--json` dumps the raw manifest for
+  scripting; `--quiet` gives a one-line `release_tag (source_revision)` form.
+  Printed automatically once at container start (`entrypoint-user.sh`, before
+  the rest of the setup output), and stays available on demand for the rest
+  of the session. Exits 1 with a short notice — rather than failing silently
+  — on images built before this file existed. Base-affecting (new
+  `rootfs/usr/local/bin/pi-devbox-version`), rebuilds `base-<hash>`.
+
 ### Changed
 
 - **Bundled `pi-toolkit` settings template: `pi-fork` balanced tier bumped to