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.

AGENTS.md

@@ -22,13 +22,16 @@ re-brand of opencode-devbox's `pi-only` variant.
   by copying `/opt/pi-extensions/skill/` over the committed `rootfs/` snapshot
   (Option 1 over Option 2 — see `skills/VENDORED.md`).
 - `entrypoint.sh` — UID/GID alignment as root, then drops to `developer`.
-- `entrypoint-user.sh` — per-container start: SSH ControlMaster socket
-  dir, LAN-access setup, MemPalace init, pi-toolkit + pi-extensions
-  deploy, mempalace-bridge symlink, fork/recall + pi-studio pi-install,
-  optional `studio-expose` bridge (when `STUDIO_EXPOSE=1`), image-baked
-  skills symlink-in, skillset deploy.
+- `entrypoint-user.sh` — per-container start: prints the `pi-devbox-version`
+  banner first (which build/commit is running, from the manifest below),
+  then SSH ControlMaster socket dir, LAN-access setup, MemPalace init,
+  pi-toolkit + pi-extensions deploy, mempalace-bridge symlink, fork/recall +
+  pi-studio pi-install, optional `studio-expose` bridge (when
+  `STUDIO_EXPOSE=1`), image-baked skills symlink-in, skillset deploy.
 - `rootfs/` — files baked into the image (bash aliases, inputrc,
-  setup-lan-access.sh, `studio-expose` helper). Also
+  setup-lan-access.sh, `studio-expose` helper, `pi-devbox-version` — wraps
+  `/etc/pi-devbox/build-manifest.json` into a human-readable summary + live
+  drift check, see README “Build provenance”). Also
   `usr/local/share/pi-devbox/skills/<name>/SKILL.md` — image-baked agent
   skills (the repo-authored `pi-devbox-environment`, plus vendored fallback
   copies of `pi-extensions` and `mempalace` — see `skills/VENDORED.md`)