feat(cli): a Makefile, a man page, and --help

Three things the project had no answer for: how to install it without
knowing dune, where to read about it, and what it does when asked.

Makefile, same shape as lectio's -- PREFIX ?= $(HOME)/.local, BINDIR,
MANDIR, and the '## '-comment help target -- so the two siblings are
driven the same way. Every recipe wraps dune in `opam exec --`, which is
the actual point of having one here: `make build` works from a plain
shell with no `eval $(opam env)` first. install goes through `dune
install` rather than a hand-rolled copy, because the binary finds its
calendar data relative to its own path; the man page is installed
separately to share/man/man1, matching lectio. install and uninstall
were both run against a scratch prefix and checked: uninstall leaves
zero files behind.

PREFIX defaults to ~/.local because that is where lectio installs and
where it actually lives on this machine, so colitur lands on an existing
PATH with no shell change. An earlier install this session went to
~/opt/colitur, which was me over-applying a rule meant for third-party
tools to one of the author's own projects; it has been removed rather
than left as a second, staler binary competing on PATH.

man/colitur.1 documents the four commands, both output formats and why
they differ, COLITUR_DATA_DIR and its refusal to fall back, the data
resolution order, exit statuses, and -- deliberately -- the limitations:
EF only, Epistle and Gospel only with the chants unbuilt and rejected
rather than guessed, and the BVM Saturday Mass-selection gap. A man page
that only lists what works is half a man page. Renders clean under
`groff -ww -z`, no warnings.

--help prints to stdout and exits 0; a usage error prints one line to
stderr and exits 2. That is the Unix convention rather than a
preference: asking for help succeeded and should be pipeable, being
invoked wrongly did not and must not pollute stdout. Both directions are
asserted in cli.t, along with a loop confirming every command the help
text advertises is one the dispatch actually accepts -- the check that
catches help drifting away from the code.

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