W14: pin K20 contains?-dict support (test-only)

contains? did not support dict key membership in the real runtime —
(contains? {:a 1} :a) threw "contains?: 2 args", contradicting its own :doc.
The fix landed (primitives.sx + sx_primitives.ml) but had no pinning test.
Add suite gate-K20-contains-dict to spec/tests/test-gate-pins.sx (4 tests,
repro from plans/sx-review/core.md): present key true, missing key false,
list membership + string substring unchanged. 8/8 green under OCaml run_tests.

Test-only: no semantics edits, no push.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/scheduled_tasks.lock

@@ -1 +1 @@
-{"sessionId":"c4d97db1-361c-4a04-a99b-c838f9385469","pid":2426590,"procStart":"349789073","acquiredAt":1780789990975}
+{"sessionId":"d510140d-6197-40b7-9bd2-125ca304ad7a","pid":697492,"procStart":"125353752","acquiredAt":1783118024555}