build: check that css template literals were not ended by a comment

A backtick inside a comment in a css`` literal ends the literal. It has
cost four sessions across three plans, it is written down in CLAUDE.md,
the skill and NOTES.md, and it was read twice in the session it then
cost a cycle in. Knowledge that has been ignored three times is not a
knowledge problem.

The expense is the report, not the mistake: the literal ends early, the
rest of the CSS parses as JavaScript, and tsc says 'Class static side
incorrectly extends base class static side' pointing at a line of prose
-- or, in a shared module, every test in the suite fails to import and
the output reads like a broken test runner. make dev-headless mean-
while keeps serving the last good bundle.

Detection is exact rather than heuristic: if a backtick in a comment
closed the literal early, the text the parser took as the literal
contains an unterminated /*. Nothing else produces that. Verified both
ways -- clean on the tree, and red on a deliberately broken comment.

.pi/skills/yellowjacket-dev/SKILL.md

@@ -97,6 +97,12 @@ reference, because you need them *before* the failure, not after.
   old behaviour. `make dev-headless` prints the esbuild error; a
   reload does not. One way to cause one is a stray backtick inside a
   comment in a `css` tagged template literal, which ends the literal.
+  **That one is a check now** — `make css-check` (instant, a pre-commit
+  hook and a CI step) names the file, the line and the cause, because
+  what you otherwise get is `Property 'scroll' does not exist on type
+  'CSSResult'` pointing at a line of prose, or every test in the suite
+  failing to import. It went in after the trap cost a fourth session in
+  which its own warning had been read twice.
 - **A failing CI job's log is reachable even when `gitea_ci job_logs`
   says it is not.** That endpoint 404s on this Gitea build. The REST
   API answers, with the `GITEA_TOKEN` already in the environment:
@@ -162,7 +168,8 @@ Two rules about climbing:
   do not exist.
 
 Before a commit, the gate is `make lint`, `make test`, `make ui-test`,
-`make bindings-check` and — from `frontend/` — `npx tsc --noEmit`. The
+`make bindings-check`, `make css-check` and — from `frontend/` —
+`npx tsc --noEmit`. The
 first four are lefthook hooks, so skipping them locally only defers the
 failure; the typecheck is a hook too but only CI runs it over the test
 tree, which is where it has actually broken.