review(회차1): emit 의 trace 메시지 단순화

회차 1 nit 반영. 'suppressing further sends would require caller
cooperation' 의 두 번째 절이 의미 불명확 — 단순 'event discarded
(best-effort send per ingest_progress contract)' 로 단축. 사실 +
영향만 기록, caller 가 어떻게 cooperate 하는지 추측은 제거.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-app/src/ingest_progress.rs

@@ -105,7 +105,7 @@ pub(crate) fn emit(
         if tx.send(event).is_err() {
             tracing::trace!(
                 target: "kebab-app::ingest_progress",
-                "progress receiver dropped; suppressing further sends would require caller cooperation"
+                "progress receiver dropped; event discarded (best-effort send per ingest_progress contract)"
             );
         }
     }