fix: preserve long run activity and large previews

Long Runtime sessions could silently stop retaining tool and Subagent activity, truncate Run history details, and reject workspace previews over 256 KB. Run history now persists in Main SQLite without the former fixed record or detail caps, surfaces possible loss from legacy caches, and renders long histories in batches.

Local and remote workspace previews now load UTF-8-safe 256 KiB pages on demand instead of rejecting larger text files. The legacy migration, clear-data path, IPC contracts, tests, and feature documentation are updated with the resulting behavior.

Release note: 长时间 Runtime 运行现在会完整保留工具和 Subagent 活动；运行记录迁移到 SQLite，并提示旧版可能已丢失的内容。超过 256KB 的本地及远程文本文件可通过分页面预览继续加载。

src/main/assistant/workspace-changes-service.ts

@@ -10,7 +10,7 @@ import {
 } from '../workspace'
 
 const MAX_DIRECTORY_ENTRIES = 500
-const MAX_PREVIEW_BYTES = 256 * 1024
+const PREVIEW_PAGE_BYTES = 256 * 1024
 const previewExtensions = new Set([
   '.c',
   '.cpp',
@@ -92,6 +92,7 @@ export class WorkspaceChangesService {
 
   async readFile(
     inputPath: string,
+    offsetBytes = 0,
     signal?: AbortSignal
   ): Promise<WorkspaceFilePreview> {
     const entry = await this.workspace.stat({
@@ -110,8 +111,9 @@ export class WorkspaceChangesService {
     }
     const preview = await this.workspace.readText({
       path: inputPath,
-      maximumBytes: MAX_PREVIEW_BYTES,
-      tooLargeMessage: '工作区文件超过 256KB 预览限制',
+      offsetBytes,
+      maximumBytes: PREVIEW_PAGE_BYTES,
+      allowTruncated: true,
       invalidUtf8Message: '工作区文件不是有效 UTF-8 文本',
       signal
     })
@@ -119,6 +121,9 @@ export class WorkspaceChangesService {
       path: preview.path,
       name: preview.name,
       content: preview.content,
+      offsetBytes: preview.offsetBytes,
+      nextOffsetBytes: preview.offsetBytes + preview.bytesRead,
+      truncated: preview.truncated,
       mimeType:
         extension === '.md' || extension === '.markdown'
           ? 'text/markdown'
@@ -177,12 +182,16 @@ export async function listWorkspaceDirectory(
 
 export async function readWorkspaceFile(
   workspace: string | WorkspaceAccess,
-  inputPath: string
+  inputPath: string,
+  offsetBytes = 0
 ): Promise<WorkspaceFilePreview> {
   if (typeof workspace !== 'string') {
-    return new WorkspaceChangesService(workspace).readFile(inputPath)
+    return new WorkspaceChangesService(workspace).readFile(
+      inputPath,
+      offsetBytes
+    )
   }
   return withLocalWorkspace(workspace, (access) =>
-    new WorkspaceChangesService(access).readFile(inputPath)
+    new WorkspaceChangesService(access).readFile(inputPath, offsetBytes)
   )
 }