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/migration.test.ts

@@ -0,0 +1,78 @@
+import { describe, expect, it } from "vitest";
+import { type FileEntry, migrateSessionEntries } from "../../src/core/session-manager.js";
+
+describe("migrateSessionEntries", () => {
+	it("should add id/parentId to v1 entries", () => {
+		const entries: FileEntry[] = [
+			{ type: "session", id: "sess-1", timestamp: "2025-01-01T00:00:00Z", cwd: "/tmp" },
+			{ type: "message", timestamp: "2025-01-01T00:00:01Z", message: { role: "user", content: "hi", timestamp: 1 } },
+			{
+				type: "message",
+				timestamp: "2025-01-01T00:00:02Z",
+				message: {
+					role: "assistant",
+					content: [{ type: "text", text: "hello" }],
+					api: "test",
+					provider: "test",
+					model: "test",
+					usage: { input: 1, output: 1, cacheRead: 0, cacheWrite: 0 },
+					stopReason: "stop",
+					timestamp: 2,
+				},
+			},
+		] as FileEntry[];
+
+		migrateSessionEntries(entries);
+
+		// Header should have version set
+		expect((entries[0] as any).version).toBe(2);
+
+		// Entries should have id/parentId
+		const msg1 = entries[1] as any;
+		const msg2 = entries[2] as any;
+
+		expect(msg1.id).toBeDefined();
+		expect(msg1.id.length).toBe(8);
+		expect(msg1.parentId).toBeNull();
+
+		expect(msg2.id).toBeDefined();
+		expect(msg2.id.length).toBe(8);
+		expect(msg2.parentId).toBe(msg1.id);
+	});
+
+	it("should be idempotent (skip already migrated)", () => {
+		const entries: FileEntry[] = [
+			{ type: "session", id: "sess-1", version: 2, timestamp: "2025-01-01T00:00:00Z", cwd: "/tmp" },
+			{
+				type: "message",
+				id: "abc12345",
+				parentId: null,
+				timestamp: "2025-01-01T00:00:01Z",
+				message: { role: "user", content: "hi", timestamp: 1 },
+			},
+			{
+				type: "message",
+				id: "def67890",
+				parentId: "abc12345",
+				timestamp: "2025-01-01T00:00:02Z",
+				message: {
+					role: "assistant",
+					content: [{ type: "text", text: "hello" }],
+					api: "test",
+					provider: "test",
+					model: "test",
+					usage: { input: 1, output: 1, cacheRead: 0, cacheWrite: 0 },
+					stopReason: "stop",
+					timestamp: 2,
+				},
+			},
+		] as FileEntry[];
+
+		migrateSessionEntries(entries);
+
+		// IDs should be unchanged
+		expect((entries[1] as any).id).toBe("abc12345");
+		expect((entries[2] as any).id).toBe("def67890");
+		expect((entries[2] as any).parentId).toBe("abc12345");
+	});
+});