Phase 15: add kept-window tests for skill-reinject:inject entries

Verify inject custom messages count as present in kept slice and are
ignored when compaction firstKeptEntryId starts after them.

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

test/kept-window.test.ts

@@ -42,6 +42,18 @@ function compactionEntry(id: string, firstKeptEntryId: string): SessionEntry {
 	} as SessionEntry;
 }
 
+function reinjectCustomMessage(id: string, content: string): SessionEntry {
+	return {
+		type: "custom_message",
+		id,
+		parentId: null,
+		timestamp: ts,
+		customType: "skill-reinject:inject",
+		content,
+		display: true,
+	} as SessionEntry;
+}
+
 const skillBlock = (name: string) =>
 	`<skill name="${name}" location="/skills/${name}/SKILL.md">\nbody\n</skill>`;
 
@@ -94,6 +106,32 @@ describe("skillsPresentInKeptWindow", () => {
 		expect(skillsPresentInKeptWindow([userMessage("k1", skillBlock("alpha"))], [])).toEqual(new Set());
 		expect(skillsPresentInKeptWindow([], ["alpha", "beta"])).toEqual(new Set());
 	});
+
+	it("detects skill blocks in skill-reinject:inject custom messages", () => {
+		const kept = [reinjectCustomMessage("inj1", skillBlock("beta"))];
+		expect(skillsPresentInKeptWindow(kept, ["alpha", "beta"])).toEqual(new Set(["beta"]));
+	});
+
+	it("ignores reinject custom outside kept slice when slicing branch", () => {
+		const branch: SessionEntry[] = [
+			reinjectCustomMessage("inj0", skillBlock("alpha")),
+			userMessage("e2", "kept start"),
+			compactionEntry("e4", "e2"),
+		];
+		const kept = getKeptEntries(branch, "e2");
+		expect(skillsPresentInKeptWindow(kept, ["alpha", "beta"])).toEqual(new Set());
+		expect(skillsPresentInKeptWindow(kept, ["alpha"])).toEqual(new Set());
+	});
+
+	it("counts reinject custom inside kept slice", () => {
+		const branch: SessionEntry[] = [
+			userMessage("e2", "kept start"),
+			reinjectCustomMessage("inj1", skillBlock("alpha")),
+			compactionEntry("e4", "e2"),
+		];
+		const kept = getKeptEntries(branch, "e2");
+		expect(skillsPresentInKeptWindow(kept, ["alpha", "beta"])).toEqual(new Set(["alpha"]));
+	});
 });
 
 describe("filterSkillsNeedingReinjectByKept", () => {