Send .git without its config; correct shallow-clone and version comments

.dockerignore lists .git/config, which holds the clone's remote URL and
any credential in it; the build stage is the final image, so it would
otherwise ship. git describe does not need it. The build-context test
asserts the entry, and the README says the image carries .git without
its config.

The README now says a shallow clone stamps a tag only when the cloned
commit itself carries one, and otherwise the short commit. The comment in
src/index.ts says a build reports the version script/build stamps into
dist/package.json, and package.json's own version only from source.

Model: opus-5-5

src/index.ts

@@ -1,5 +1,7 @@
-// package.json is the one place the version is written. tsc copies it to
-// dist/package.json, so this path resolves from source and from dist/src/.
+// A build reports the version script/build stamps into dist/package.json;
+// package.json's own version is reported only when running from source. tsc
+// copies package.json to dist/package.json, so this path resolves from source
+// and from dist/src/.
 import pkg from "../package.json" with { type: "json" };
 
 export const VERSION: string = pkg.version;