fix(session): repaired torn JSONL appends

Rolled partial file appends back to their pre-write size and marked malformed resumed sessions for an atomic rewrite.

Retried transient persistence failures from in-memory state and surfaced the first failure in the interactive TUI.

Fixes #8596

packages/coding-agent/test/session-loader-stream.test.ts

@@ -92,6 +92,7 @@ describe("loadEntriesFromFileStream (Bun.JSONL parity)", () => {
 
 		expect(titleSlot?.title).toBe("Visitor");
 		expect(entryIds(visited)).toEqual(["s1", "m1", "m2"]);
+		expect((await sessionLoader.loadEntriesFromFileStream(file)).malformedRecords).toBe(1);
 	});
 	it("does not revisit entries before a malformed line spanning stream chunks", async () => {
 		const content = [
@@ -167,6 +168,7 @@ describe("loadEntriesFromFileStream (Bun.JSONL parity)", () => {
 		expect(entryTypes(stream.entries)).toEqual(["session", "message", "message"]);
 		const ids = messageIds(stream.entries);
 		expect(ids).toEqual(["m1", "m2"]); // valid entries kept in order, malformed skipped
+		expect(stream.malformedRecords).toBe(1);
 	});
 
 	it("matches parseSessionContent when there is no title slot (header is the first line)", async () => {
@@ -217,5 +219,6 @@ describe("loadEntriesFromFileStream (Bun.JSONL parity)", () => {
 		const stream = await sessionLoader.loadEntriesFromFileStream(missing);
 		expect(stream.entries).toEqual([]);
 		expect(stream.titleSlot).toBeUndefined();
+		expect(stream.malformedRecords).toBe(0);
 	});
 });