fix: allow merging regular user messages into summary in API shaping

The mergeConsecutiveApiMessages function now allows merging a regular
user message that follows a summary message. Since this is API-only
shaping (storage is unaffected), rewind semantics remain intact.

- Remove !prev.isSummary guard to allow merge INTO summary
- Keep !msg.isSummary guard to prevent merging a summary INTO something
- Update tests to reflect new expected behavior

Author: hannesrudolph

src/core/task/__tests__/mergeConsecutiveApiMessages.spec.ts

@@ -19,12 +19,28 @@ describe("mergeConsecutiveApiMessages", () => {
 		expect(merged[1].role).toBe("assistant")
 	})
 
-	it("does not merge summary messages", () => {
+	it("merges regular user message into a summary (API shaping only)", () => {
 		const merged = mergeConsecutiveApiMessages([
 			{ role: "user", content: [{ type: "text", text: "Summary" }], ts: 1, isSummary: true, condenseId: "s" },
 			{ role: "user", content: [{ type: "text", text: "After" }], ts: 2 },
 		])
 
+		expect(merged).toHaveLength(1)
+		expect(merged[0].isSummary).toBe(true)
+		expect(merged[0].content).toEqual([
+			{ type: "text", text: "Summary" },
+			{ type: "text", text: "After" },
+		])
+	})
+
+	it("does not merge a summary into a preceding message", () => {
+		const merged = mergeConsecutiveApiMessages([
+			{ role: "user", content: [{ type: "text", text: "Before" }], ts: 1 },
+			{ role: "user", content: [{ type: "text", text: "Summary" }], ts: 2, isSummary: true, condenseId: "s" },
+		])
+
 		expect(merged).toHaveLength(2)
+		expect(merged[0].isSummary).toBeUndefined()
+		expect(merged[1].isSummary).toBe(true)
 	})
 })

src/core/task/mergeConsecutiveApiMessages.ts

@@ -34,8 +34,8 @@ export function mergeConsecutiveApiMessages(messages: ApiMessage[], options?: {
 			prev &&
 			prev.role === msg.role &&
 			mergeRoles.has(msg.role) &&
-			// Keep summary/truncation markers isolated so rewind semantics stay clear.
-			!prev.isSummary &&
+			// Allow merging regular messages into a summary (API-only shaping),
+			// but never merge a summary into something else.
 			!msg.isSummary &&
 			!prev.isTruncationMarker &&
 			!msg.isTruncationMarker