Add Makefile shims for cibuild and precommit (closes #34)

script/cibuild and script/precommit both existed and were already the
documented CI and pre-commit entrypoints, but neither had a Makefile
target, so the standing rule to drive the repo through make targets
rather than the underlying tool could not be followed for either.

It matters most for the build. A bare `docker build .` fails closed on
the CHECK_EPOCH guard by design, so script/cibuild is one of only three
supported ways to build an image here, and it was the only one of the
three without a target while `make docker` had one.

The two targets are thin shims in the same style as every other target
and change nothing about what the scripts do. .PHONY was already
complete for the targets that existed and now lists both new ones.

README.md's Entrypoints section gains the script-to-target mapping so
the two documents agree, including the two names that do not match:
script/install-precommit is `make hooks`, and script/precommit is
`make precommit`. It also notes that `make cibuild` is the slowest
target, because it is the only one that runs two container builds --
while still taking seconds once the shared script/bootstrap layer is
cached, since Dockerfile.lint's first four instructions are
byte-identical to the main Dockerfile's and the pinned-Hugo compile is
therefore paid once per machine rather than twice.

Two accuracy fixes to text the same section already carried. The
script/install-precommit bullet said the installed hook runs
script/check; the script writes script/precommit into
.git/hooks/pre-commit, and its own header comment says so. And the
Makefile is described as listing the operations you are expected to run
rather than as the authoritative list of everything the repo can do,
which is not literally true: script/projectname is an internal helper
that script/docker calls to compute a tag, and it has no target
deliberately -- a target for it would be noise in the `make<tab>`
listing this change exists to make useful.

TODO.md also loses the stale Future Step asking someone to confirm the
static/_headers file took effect in production. That was confirmed live
on both hostnames on 2026-08-10 and recorded at
#14 , so the item is work
already done. The Status paragraph's matching "unverified in production
until the next deploy" clause is corrected for the same reason: a commit
that edits TODO.md should not leave a known-false statement in it.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: bootstrap setup test lint fmt fmt-check check docker hooks serve
+.PHONY: bootstrap setup test lint fmt fmt-check check docker cibuild precommit hooks serve
 
 bootstrap:
 	@script/bootstrap
@@ -24,6 +24,12 @@ check:
 docker:
 	@script/docker
 
+cibuild:
+	@script/cibuild
+
+precommit:
+	@script/precommit
+
 hooks:
 	@script/install-precommit