Phase 5: add getKeptEntries — branch slice from firstKeptEntryId.

Implements kept-window boundary per SPEC §6.4 for dedup before re-inject.

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

src/kept.ts

@@ -0,0 +1,10 @@
+import type { SessionEntry } from "@earendil-works/pi-coding-agent";
+
+/** Branch slice from firstKeptEntryId through tail (SPEC §6.4). */
+export function getKeptEntries(branch: SessionEntry[], firstKeptEntryId: string): SessionEntry[] {
+	const startIndex = branch.findIndex((entry) => entry.id === firstKeptEntryId);
+	if (startIndex < 0) {
+		return [];
+	}
+	return branch.slice(startIndex);
+}