Phase 15: add ensureCompactionSourceMarked for compaction source fallback

Extract shared helper so session_before_compact and session_compact can
both infer auto when compaction was not explicitly manual.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/compaction.ts

@@ -25,13 +25,18 @@ export function markManualCompactionFromInput(text: string, runtime: CompactionR
 	}
 }
 
-/** session_before_compact: default to auto unless input already marked manual (SPEC §8). */
-export function markAutoCompactionBeforeCompact(runtime: CompactionRuntime): void {
+/** If not explicitly manual, mark compaction source as auto (SPEC §8). */
+export function ensureCompactionSourceMarked(runtime: CompactionRuntime): void {
 	if (runtime.pendingCompactionSource !== "manual") {
 		runtime.pendingCompactionSource = "auto";
 	}
 }
 
+/** session_before_compact: default to auto unless input already marked manual (SPEC §8). */
+export function markAutoCompactionBeforeCompact(runtime: CompactionRuntime): void {
+	ensureCompactionSourceMarked(runtime);
+}
+
 /** Gate re-inject on session_compact from enabled layer and compaction source (SPEC §8). */
 export function shouldReinjectAfterCompaction(
 	sessionOverride: boolean | null,