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>

.dockerignore

@@ -1,9 +1,12 @@
 # Mirrors .gitignore, with one deliberate exception: .gitignore itself stays
 # in the build context, because prettier 3 reads it as a default ignore file
 # and dropping it would change what the lint phase's prettier check sees.
-
-# VCS
-.git
+#
+# .git is deliberately NOT excluded: the build derives the version it stamps
+# from it (script/version). It is sent without its config, which holds the
+# clone's remote URL and any credential in it, and which the build stage, the
+# final image, would otherwise carry. git describe does not need it.
+.git/config
 
 # OS
 .DS_Store