Phase 8: mark auto compaction before_compact — default source unless manual.

session_before_compact sets pendingCompactionSource to auto when input did not mark /compact.

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

src/compaction.ts

@@ -17,3 +17,10 @@ export function markManualCompactionFromInput(text: string, runtime: CompactionR
 		runtime.pendingCompactionSource = "manual";
 	}
 }
+
+/** session_before_compact: default to auto unless input already marked manual (SPEC §8). */
+export function markAutoCompactionBeforeCompact(runtime: CompactionRuntime): void {
+	if (runtime.pendingCompactionSource !== "manual") {
+		runtime.pendingCompactionSource = "auto";
+	}
+}