Derive the image version from git; send .git to the build (closes #69)

The canonical documents told every repo to exclude .git from the build
context, default ARG VERSION to dev and never run git describe in a
build stage, so an image built from a clone with no build argument
reported dev. .dockerignore now sends .git. The Dockerfile example in
REPO_POLICIES.md installs git, takes the VERSION build argument when one
is given and otherwise git describe --tags --always, and fails when .git
is present but the version is empty, dev or unknown. The checklists, the
Go docs' Makefile comments, the README, and this repo's Dockerfile and
scripts now say the same.

Model: opus-5-5

README.md

@@ -132,8 +132,7 @@ alpine. We provide:
   `script/check`, compute `version` from `git describe`, then
   `docker build --no-cache --build-arg VERSION="$version" -t prompts .` (what CI
   runs; it bootstraps because CI checks out and runs this alone while
-  `script/fmt-check` is native, and the version is computed on the host because
-  `.dockerignore` excludes `.git`)
+  `script/fmt-check` is native)
 - `script/precommit` — run by the git pre-commit hook (our own extension); calls
   `script/check`
 - `script/install-precommit` — installs the git pre-commit hook (our own