fix(gates): close six PR #135 review findings in gates and their tests

B1: check-skill-version-bump.sh resolves every merge-base with `git merge-base
--all` instead of the single base git happens to pick. A criss-cross history has
two, so the verdict turned on that choice: a skill byte-identical to main's tip
could still be reported "not above merge-base" / "not above main tip" and fail a
push that should pass. A skill now counts as changed only when it differs from
EVERY base, and its version must exceed the version at every base it exists at
as well as at the main tip; with more than one base the failure names which one.
Case 40 in tests/test-skill-version-bump.sh builds the criss-cross fixture and
pins both directions.

B2: check-apm-current.sh no longer assumes the remote default branch is `main`
when origin/HEAD is unset. A checkout whose default is `master` was standing on
its default branch and being told "this is a feature branch, so discard it" --
to throw away a real lock update. With origin/HEAD unset nothing is asserted and
the neutral advice stands. tests/test-apm-current-hook.sh covers the unset case
on both `main` and `master`.

#4: the required-frontmatter checks folded into skill-size-check.sh by c8a7c9e
were untested apart from the leading-zero shape -- mutating the missing-version
ERROR into a no-op left every suite green. tests/test-adr0020-frontmatter.sh now
pins name presence and non-emptiness, metadata.version presence and semver
shape, and the four grep defects the deleted test-skill-frontmatter.sh named.

#5: nothing asked whether a Vale rule still MATCHES anything -- rewriting
CompositionNote.yml's tokens to match nothing left test-vale-wrap.sh at 63/63.
Case 35 enumerates the rule files under the Kyberforge* style directories at run
time, requires an alert from each on its own fixture, and fails when a
discovered rule has no fixture row. The stale comment at case 31 is corrected.

#6: tests/run-tests.sh --strict exited 0 when discovery found no test-*.sh at
all; strictness only ever acted on skips, and with no suites there were none. It
now cross-checks the git index the way run-bats.sh does and fails
unconditionally on an empty set, naming the search root.

N9: the skill-size-check hook description in .pre-commit-config.yaml covered
only the size, context-budget and boundary-target gates. It now also names the
required frontmatter fields, matching docs/spec/gates.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NwD8Egs5r4ndqeFLmhusX2

plugins/kyberforge/.apm/hooks/check-apm-current.sh

@@ -56,12 +56,21 @@ emit() {
 # is churn unrelated to the branch and should be discarded. The branch name only
 # selects between fixed strings and is never interpolated. Outside a git checkout,
 # or on a detached HEAD, the neutral advice stands.
+#
+# So does an UNSET origin/HEAD, which is the common state: git only writes it on
+# clone, and `git remote add` never does. The fallback here used to be `main`,
+# which is a guess, and it is wrong in exactly the repos that would notice — a
+# checkout whose default branch is `master` was told "this is a feature branch,
+# so discard it" while standing on its default branch, i.e. told to throw away a
+# real lock update. There is no cheap way to learn the remote's default without
+# the network, so nothing is asserted: the advice stays neutral and the reader
+# decides.
 lock_advice="commit it or discard it deliberately."
 current_branch="$(git symbolic-ref --short -q HEAD 2> /dev/null || true)"
-if [[ -n "$current_branch" ]]; then
-  default_branch="$(git symbolic-ref --short -q refs/remotes/origin/HEAD 2> /dev/null || true)"
-  default_branch="${default_branch#origin/}"
-  if [[ "$current_branch" == "${default_branch:-main}" ]]; then
+default_branch="$(git symbolic-ref --short -q refs/remotes/origin/HEAD 2> /dev/null || true)"
+default_branch="${default_branch#origin/}"
+if [[ -n "$current_branch" && -n "$default_branch" ]]; then
+  if [[ "$current_branch" == "$default_branch" ]]; then
     lock_advice="this is the default branch, so commit it or discard it deliberately."
   else
     lock_advice="this is a feature branch, so discard it: git checkout -- apm.lock.yaml && apm install"