docker build . stamps the git tag or short commit, not dev (closes #154)

A plain `docker build .` now stamps the version from git rather than `dev`/`0.0.0`. After `tsc`, `script/build` writes into `dist/package.json` the version `script/version` decides: `VERSION` when given, otherwise `git describe --tags --always` (the tag; or tag, commits since and short commit; or the short commit), otherwise `package.json`'s. A checkout with `.git` that yields an empty, `dev` or `unknown` version fails the build. `.dockerignore` sends `.git` but not `.git/config`, so no remote URL or credential reaches the image. `ARG VERSION` has no default, and the host scripts' version still wins.

Not changed: `REPO_POLICIES.md` still says `ARG VERSION=dev` until the shared policy changes.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

script/cibuild

@@ -15,9 +15,9 @@ main() {
     cd "$ROOT"
     # Own line: a failing command substitution inside an argument does
     # not trip `set -e`, so the inline form degrades silently to an
-    # empty constant. VERSION is computed here because .dockerignore
-    # excludes .git, so `git describe` in a build stage yields an empty
-    # version without failing.
+    # empty constant. The version resolved here goes in as the VERSION
+    # build arg, which takes precedence over what the build would derive
+    # from the .git in its context.
     version="$(git describe --tags --always --dirty 2>/dev/null || true)"
     [ -n "$version" ] || version="unknown"
     docker build --no-cache \