feat(cli): --version, a CHANGELOG, and a release target

Groundwork for tagging. The project had no version anywhere: not in
dune-project, not in the binary, no CHANGELOG, no tags.

The version lives in two places -- dune-project, which generates
colitur.opam, and a constant in bin/main.ml, which is what --version
prints. Two rather than one because dune's watermarking only substitutes
in a release tarball, so a binary built the ordinary way from a checkout
would report a placeholder. The release target rewrites both and then
re-checks both, and finally requires the freshly built binary to report
the version it just wrote: a release that bumped one and not the other
would ship a binary disagreeing with its own package metadata.

--version is deliberately not embedded in the help text. cli.t pins
help's first line, and a version there would mean editing a test
expectation every release for no gain.

The release target mirrors lectio's, refusals included: no release from a
dirty tree, none without a CHANGELOG entry for that version, none whose
version bump silently failed to apply, and `make check` -- the full
1583-9999 sweep, not the sampling suite -- must pass before the tag is
created.

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