Refactor session manager: migration chain, validation, tests

- Add migrateV1ToV2/migrateToCurrentVersion for extensible migrations
- createSummaryMessage now takes timestamp from entry
- loadEntriesFromFile validates session header
- findMostRecentSession only returns valid session files (reads first 512 bytes)
- Remove ConversationEntry alias
- Fix mom context.ts TreeNode type

Tests:
- migration.test.ts: v1 migration, idempotency
- build-context.test.ts: 14 tests covering trivial, compaction, branches
- file-operations.test.ts: loadEntriesFromFile, findMostRecentSession

packages/coding-agent/test/session-manager/file-operations.test.ts

@@ -0,0 +1,127 @@
+import { mkdirSync, rmSync, writeFileSync } from "fs";
+import { tmpdir } from "os";
+import { join } from "path";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { findMostRecentSession, loadEntriesFromFile } from "../../src/core/session-manager.js";
+
+describe("loadEntriesFromFile", () => {
+	let tempDir: string;
+
+	beforeEach(() => {
+		tempDir = join(tmpdir(), `session-test-${Date.now()}`);
+		mkdirSync(tempDir, { recursive: true });
+	});
+
+	afterEach(() => {
+		rmSync(tempDir, { recursive: true, force: true });
+	});
+
+	it("returns empty array for non-existent file", () => {
+		const entries = loadEntriesFromFile(join(tempDir, "nonexistent.jsonl"));
+		expect(entries).toEqual([]);
+	});
+
+	it("returns empty array for empty file", () => {
+		const file = join(tempDir, "empty.jsonl");
+		writeFileSync(file, "");
+		expect(loadEntriesFromFile(file)).toEqual([]);
+	});
+
+	it("returns empty array for file without valid session header", () => {
+		const file = join(tempDir, "no-header.jsonl");
+		writeFileSync(file, '{"type":"message","id":"1"}\n');
+		expect(loadEntriesFromFile(file)).toEqual([]);
+	});
+
+	it("returns empty array for malformed JSON", () => {
+		const file = join(tempDir, "malformed.jsonl");
+		writeFileSync(file, "not json\n");
+		expect(loadEntriesFromFile(file)).toEqual([]);
+	});
+
+	it("loads valid session file", () => {
+		const file = join(tempDir, "valid.jsonl");
+		writeFileSync(
+			file,
+			'{"type":"session","id":"abc","timestamp":"2025-01-01T00:00:00Z","cwd":"/tmp"}\n' +
+				'{"type":"message","id":"1","parentId":null,"timestamp":"2025-01-01T00:00:01Z","message":{"role":"user","content":"hi","timestamp":1}}\n',
+		);
+		const entries = loadEntriesFromFile(file);
+		expect(entries).toHaveLength(2);
+		expect(entries[0].type).toBe("session");
+		expect(entries[1].type).toBe("message");
+	});
+
+	it("skips malformed lines but keeps valid ones", () => {
+		const file = join(tempDir, "mixed.jsonl");
+		writeFileSync(
+			file,
+			'{"type":"session","id":"abc","timestamp":"2025-01-01T00:00:00Z","cwd":"/tmp"}\n' +
+				"not valid json\n" +
+				'{"type":"message","id":"1","parentId":null,"timestamp":"2025-01-01T00:00:01Z","message":{"role":"user","content":"hi","timestamp":1}}\n',
+		);
+		const entries = loadEntriesFromFile(file);
+		expect(entries).toHaveLength(2);
+	});
+});
+
+describe("findMostRecentSession", () => {
+	let tempDir: string;
+
+	beforeEach(() => {
+		tempDir = join(tmpdir(), `session-test-${Date.now()}`);
+		mkdirSync(tempDir, { recursive: true });
+	});
+
+	afterEach(() => {
+		rmSync(tempDir, { recursive: true, force: true });
+	});
+
+	it("returns null for empty directory", () => {
+		expect(findMostRecentSession(tempDir)).toBeNull();
+	});
+
+	it("returns null for non-existent directory", () => {
+		expect(findMostRecentSession(join(tempDir, "nonexistent"))).toBeNull();
+	});
+
+	it("ignores non-jsonl files", () => {
+		writeFileSync(join(tempDir, "file.txt"), "hello");
+		writeFileSync(join(tempDir, "file.json"), "{}");
+		expect(findMostRecentSession(tempDir)).toBeNull();
+	});
+
+	it("ignores jsonl files without valid session header", () => {
+		writeFileSync(join(tempDir, "invalid.jsonl"), '{"type":"message"}\n');
+		expect(findMostRecentSession(tempDir)).toBeNull();
+	});
+
+	it("returns single valid session file", () => {
+		const file = join(tempDir, "session.jsonl");
+		writeFileSync(file, '{"type":"session","id":"abc","timestamp":"2025-01-01T00:00:00Z","cwd":"/tmp"}\n');
+		expect(findMostRecentSession(tempDir)).toBe(file);
+	});
+
+	it("returns most recently modified session", async () => {
+		const file1 = join(tempDir, "older.jsonl");
+		const file2 = join(tempDir, "newer.jsonl");
+
+		writeFileSync(file1, '{"type":"session","id":"old","timestamp":"2025-01-01T00:00:00Z","cwd":"/tmp"}\n');
+		// Small delay to ensure different mtime
+		await new Promise((r) => setTimeout(r, 10));
+		writeFileSync(file2, '{"type":"session","id":"new","timestamp":"2025-01-01T00:00:00Z","cwd":"/tmp"}\n');
+
+		expect(findMostRecentSession(tempDir)).toBe(file2);
+	});
+
+	it("skips invalid files and returns valid one", async () => {
+		const invalid = join(tempDir, "invalid.jsonl");
+		const valid = join(tempDir, "valid.jsonl");
+
+		writeFileSync(invalid, '{"type":"not-session"}\n');
+		await new Promise((r) => setTimeout(r, 10));
+		writeFileSync(valid, '{"type":"session","id":"abc","timestamp":"2025-01-01T00:00:00Z","cwd":"/tmp"}\n');
+
+		expect(findMostRecentSession(tempDir)).toBe(valid);
+	});
+});