docs(scripts): record that the bare-arrow carve-out is now unexercised

The comment justified the ADR-0020 compressed-form gate with diagnose's
process chain 'fix -> regression-test', which without the gate read as a
route to a non-existent regression-test skill. Issue #99 cut that chain
when it retrofitted the description, so the gate now produces an
identical verdict corpus-wide whether it is applied or not.

Keeping the branch. It guards against prose no one has written yet, any
new process chain re-arms it, and the bare-arrow rule it sits on is the
sole extractor for three real targets in kyberforge's audit skills, all
written unbackticked. Unexercised is not the same as unnecessary — the
comment just needed to stop citing evidence that no longer exists.

Refs #99

scripts/skill-size-check.sh

@@ -456,8 +456,17 @@ def known_targets(start_dir):
 #     condition, pc-run's "run pre-commit hooks" reads as a route to a
 #     non-existent `pre-commit` skill.
 #   * A BARE arrow target counts only in ADR-0020's compressed boundary form,
-#     `Not <thing> -> <skill-name>`. Without that, diagnose's process chain
-#     "fix -> regression-test" reads as a route to `regression-test`.
+#     `Not <thing> -> <skill-name>`. The example that motivated it is gone:
+#     diagnose's process chain "fix -> regression-test", which without the
+#     gate read as a route to a non-existent `regression-test` skill, was cut
+#     when issue #99 retrofitted that description. So the gate is currently
+#     UNEXERCISED — gating and not gating produce the same verdict corpus-wide.
+#     Keep it anyway. It is a false-positive guard against prose no one has
+#     written yet, and any new process chain re-arms it. Unexercised is not the
+#     same as unnecessary, and the branch it guards is still load-bearing: the
+#     bare-arrow rule is the sole extractor for three real targets in
+#     kyberforge's audit skills (agent-audit -> agent-author, agent-audit ->
+#     skill-audit, skill-audit -> skill-author), all written unbackticked.
 #   * A backticked hyphenated token counts only inside a boundary sentence.
 #     Unconditionally, `pre-push` or `commit-msg` in a TRIGGER clause is a hard
 #     FAIL with no escape hatch. Gating it costs nothing (measured over this