docs(scheduler-core): mark task 12.5 complete with fresh verification evidence

Task 12.5 (REPL live-operator-testing fixes) was implemented and its bugs
fixed in an earlier commit, but the tasks.md checkbox was never flipped and
carried no _Verified: line, unlike every other completed task in this spec.
Ran /kiro-verify-completion against it with fresh evidence rather than
trusting the earlier self-report: fresh build+full suite (41/42, only the
pre-existing unrelated bug-1560 failing), a live subprocess smoke test of
the real xce_scheduler binary for all five amendment gaps (including a
hand-built .replay fixture with resource data to close the one gap the
first pass left unit-tested-only), and direct source inspection of each
fix. All five confirmed correct; checkbox flipped, _Verified: line added.

.kiro/specs/scheduler-core/spec.json

@@ -1,7 +1,7 @@
 {
   "feature_name": "scheduler-core",
   "created_at": "2026-07-27T00:00:00Z",
-  "updated_at": "2026-08-07T00:00:00Z",
+  "updated_at": "2026-08-19T00:00:00Z",
   "language": "en",
   "phase": "implementation-complete",
   "parent_spec": "scheduler-framework",