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>

plans/agent-briefings/sx-gate-loop.md

@@ -45,7 +45,7 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 `spec/tests/test-gate-pins.sx` (one `defsuite` per finding).
 
 - [x] K18 [W7] — `expt` overflow now float-promotes (no 63-bit wrap)
-- [ ] K20 [W7] — identify the landed W7 fix and pin it
+- [x] K20 [W7] — `contains?` now supports dict key membership
 - [ ] K09/K11/K39 [W5] — landed special-form fixes, pin each
 - [ ] K49 [W8] — render depth/cycle guard (infinite recursive component)
 - [ ] crit-2 [W1] — signal-return frame key (verify the pin is non-vacuous)
@@ -77,6 +77,13 @@ Pin each confirmed-and-fixed finding with a minimal repro. Add suites to
 
 ## Progress log (newest first)
 
+- 2026-07-03 — **K20 contains?-dict pin (item A.2)**. Mapped K-codes by
+  core.md severity order (K17 append!, K18 expt, K19 harness-drift, K20
+  contains?-dict). Added suite `gate-K20-contains-dict` to
+  `spec/tests/test-gate-pins.sx` (4 tests): present dict key → true, missing
+  key → false, list membership unchanged, string substring unchanged. Repro
+  from core.md ("(contains? {:a 1} :a) threw `contains?: 2 args`"). 8/8 green
+  across both suites under OCaml run_tests. Test-only.
 - 2026-07-03 — **K18 expt-overflow pin (item A.1)**. Bootstrapped this briefing
   from PLAN.md §W14 (the referenced file did not exist yet). Added
   `spec/tests/test-gate-pins.sx` with suite `gate-K18-expt-overflow` (4 tests):