perf: skip pre-compression history on session resume

On resume (-r), the CLI was loading and replaying the entire session
recording, including messages that had already been compressed away.
For long-running Forever Mode sessions this made resume extremely slow.

Add lastCompressionIndex to ConversationRecord, stamped when
compression succeeds. On resume, only messages from that index
onward are loaded into the client history and UI. Fully backward
compatible — old sessions without the field load all messages as before.

packages/cli/src/utils/sessionUtils.test.ts

@@ -10,10 +10,16 @@ import {
   extractFirstUserMessage,
   formatRelativeTime,
   hasUserOrAssistantMessage,
+  convertSessionToHistoryFormats,
   SessionError,
 } from './sessionUtils.js';
-import type { Config, MessageRecord } from '@google/gemini-cli-core';
+import type {
+  Config,
+  MessageRecord,
+  ConversationRecord,
+} from '@google/gemini-cli-core';
 import { SESSION_FILE_PREFIX } from '@google/gemini-cli-core';
+import { MessageType } from '../ui/types.js';
 import * as fs from 'node:fs/promises';
 import path from 'node:path';
 import { randomUUID } from 'node:crypto';
@@ -765,3 +771,80 @@ describe('formatRelativeTime', () => {
     expect(formatRelativeTime(thirtySecondsAgo.toISOString())).toBe('Just now');
   });
 });
+
+describe('convertSessionToHistoryFormats', () => {
+  const messages: ConversationRecord['messages'] = [
+    {
+      id: '1',
+      type: 'user',
+      timestamp: '2024-01-01T10:00:00Z',
+      content: 'First question',
+    },
+    {
+      id: '2',
+      type: 'gemini',
+      timestamp: '2024-01-01T10:01:00Z',
+      content: 'First answer',
+    },
+    {
+      id: '3',
+      type: 'user',
+      timestamp: '2024-01-01T10:02:00Z',
+      content: 'Second question',
+    },
+    {
+      id: '4',
+      type: 'gemini',
+      timestamp: '2024-01-01T10:03:00Z',
+      content: 'Second answer',
+    },
+  ];
+
+  it('should convert all messages when startIndex is undefined', () => {
+    const { uiHistory } = convertSessionToHistoryFormats(messages);
+
+    expect(uiHistory).toHaveLength(4);
+    expect(uiHistory[0]).toEqual({
+      type: MessageType.USER,
+      text: 'First question',
+    });
+    expect(uiHistory[1]).toEqual({
+      type: MessageType.GEMINI,
+      text: 'First answer',
+    });
+    expect(uiHistory[2]).toEqual({
+      type: MessageType.USER,
+      text: 'Second question',
+    });
+    expect(uiHistory[3]).toEqual({
+      type: MessageType.GEMINI,
+      text: 'Second answer',
+    });
+  });
+
+  it('should show only post-compression messages with a leading info message when startIndex is provided', () => {
+    const { uiHistory } = convertSessionToHistoryFormats(messages, 2);
+
+    // Should have: 1 info message + 2 remaining messages
+    expect(uiHistory).toHaveLength(3);
+
+    // First item is the compression info message
+    expect(uiHistory[0].type).toBe(MessageType.INFO);
+    expect((uiHistory[0] as { type: string; text: string }).text).toContain(
+      '2 messages',
+    );
+    expect((uiHistory[0] as { type: string; text: string }).text).toContain(
+      'compressed',
+    );
+
+    // Remaining items are the post-compression messages
+    expect(uiHistory[1]).toEqual({
+      type: MessageType.USER,
+      text: 'Second question',
+    });
+    expect(uiHistory[2]).toEqual({
+      type: MessageType.GEMINI,
+      text: 'Second answer',
+    });
+  });
+});