docs: fix quick-start — bare 'run devbox' lands in a shell, not opencode

The image's default CMD is bash -l, so 'docker compose run --rm devbox'
with no command drops into a login shell; you pass 'opencode' explicitly to
start the harness. The README quick-start claimed the opposite and carried a
garbled 'Use bash instead of (no command)' half-sentence; the same error was
mirrored in the Hub HUB_TEMPLATE. Fix both and regenerate DOCKER_HUB.md.
Doc-only — no image bytes change.

CHANGELOG.md

@@ -8,6 +8,15 @@ Tags follow `v{opencode_version}[letter]` — bare tag for the first build on a
 
 ## Unreleased
 
+### Docs (no image change)
+
+- Fix the quick-start description in `README.md` and the Hub `HUB_TEMPLATE`
+  (`scripts/generate-dockerhub-md.py`, regenerated `DOCKER_HUB.md`): bare
+  `docker compose run --rm devbox` lands in a **login shell** (default `CMD` is
+  `bash -l`), not opencode. The old copy claimed the opposite and had a garbled
+  "Use `bash` instead of (no command)" half-sentence. Pass `opencode` explicitly
+  to start the harness directly. Doc-only — does not trigger a new image build.
+
 ## v2.1.0 — 2026-06-13
 
 Image-semver **minor**: adds the OMOS bundled-skills image-symlink mechanism and