fix: reduce activity history disk writes during streaming

Save only changed activity records instead of rewriting the entire history snapshot. Migrate existing history to schema 44 while preserving order and duplicate IDs.

Run unit tests with up to four workers and keep the configured integration phase serial. Include activity-history I/O, storage-upgrade, settings, and OpenCode isolation regression coverage.

Validation: 4 related test files / 123 tests passed; typecheck and lint passed.

Release note: Reduce disk writes while agent activity updates stream in, preserving existing activity history.

src/renderer/src/SettingsPanel.test.tsx

@@ -4403,6 +4403,57 @@ describe('SettingsPanel runtime files', () => {
     expect(headers).toHaveValue('{"Authorization":"replacement"}')
   })
 
+  it.each([false, true])('resolves a fresh persisted store after adding a text connection (default: %s)', async (makeDefault) => {
+    const { mkdtemp, rm } = await import('node:fs/promises')
+    const { tmpdir } = await import('node:os')
+    const { join } = await import('node:path')
+    // Load main-process code only in this integration test, outside the web TS project.
+    const storeModule = '../../main/runtime-settings-store'
+    const selectionModule = '../../main/agent/runtime-selection'
+    const { RuntimeSettingsStore } = await import(storeModule)
+    const { applyRuntimeSelection } = await import(selectionModule)
+    const root = await mkdtemp(join(tmpdir(), 'goodbuddy-fresh-selection-'))
+    const cipher = {
+      isAvailable: () => false,
+      encrypt: () => { throw new Error('No credentials in this test') },
+      decrypt: () => { throw new Error('No credentials in this test') }
+    }
+    const path = join(root, 'runtime-settings.json')
+    const store = new RuntimeSettingsStore(path, cipher, {})
+    try {
+      const fresh = await store.getPublicSettings()
+      expect(fresh.opencodeModelSource).toEqual({ kind: 'default' })
+      expect(fresh.modelName).toBe('qwen3')
+      getRuntime.mockResolvedValueOnce(fresh)
+      updateRuntime.mockImplementationOnce((input) => store.update(input))
+      render(<SettingsPanel {...heartbeatSettingsProps} open
+        onClearLocalData={vi.fn(async () => {})} onClose={vi.fn()} onSaved={vi.fn()} />)
+      fireEvent.click(screen.getByRole('tab', { name: '模型连接' }))
+      await screen.findByDisplayValue('qwen3')
+      fireEvent.click(screen.getByRole('button', { name: '添加自定义' }))
+      fireEvent.change(screen.getByLabelText('模型接口 URL'), {
+        target: { value: 'http://127.0.0.1:12345/v1' }
+      })
+      fireEvent.change(screen.getByLabelText('模型'), { target: { value: 'isolated-new-model' } })
+      if (makeDefault) fireEvent.click(screen.getByRole('radio', { name: '默认连接' }))
+      fireEvent.click(screen.getByRole('button', { name: '保存设置' }))
+      await waitFor(() => expect(updateRuntime).toHaveBeenCalled())
+      await updateRuntime.mock.results[0]!.value
+      const reloaded = new RuntimeSettingsStore(path, cipher, {})
+      const selected = applyRuntimeSelection(await reloaded.getResolvedSettings(), { provider: 'opencode' })
+      expect(selected.settings.opencodeEmbedded).toBe(true)
+      expect(selected.settings.opencodeModelProfile).toMatchObject({
+        modelName: makeDefault ? 'isolated-new-model' : 'qwen3',
+        baseUrl: makeDefault ? 'http://127.0.0.1:12345/v1' : 'http://127.0.0.1:11434/v1',
+        authentication: 'none'
+      })
+      expect((await reloaded.getPublicSettings()).opencodeModelSource).toEqual({ kind: 'default' })
+    } finally {
+      cleanup()
+      await rm(root, { recursive: true, force: true })
+    }
+  })
+
   it('keeps saved Runtime sources valid when defaulting a new text profile', async () => {
     render(
       <SettingsPanel