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.test.ts

@@ -95,7 +95,10 @@ describe('workspace file browsing', () => {
       path: 'remote.md',
       name: 'remote.md',
       content: '# fake\n',
-      size: 7
+      size: 7,
+      offsetBytes: 0,
+      bytesRead: 7,
+      truncated: false
     }))
     const getChanges = vi.fn(async () => ({
       rootPath: '/remote',
@@ -136,7 +139,9 @@ describe('workspace file browsing', () => {
     expect(readText).toHaveBeenCalledWith(
       expect.objectContaining({
         path: 'remote.md',
-        maximumBytes: 256 * 1024
+        offsetBytes: 0,
+        maximumBytes: 256 * 1024,
+        allowTruncated: true
       })
     )
   })
@@ -171,15 +176,11 @@ describe('workspace file browsing', () => {
     })
   })
 
-  it('rejects traversal, unsupported files, invalid UTF-8, and oversized files', async () => {
+  it('rejects traversal, unsupported files, and invalid UTF-8', async () => {
     const directory = await mkdtemp(join(tmpdir(), 'goodbuddy-files-'))
     temporaryDirectories.push(directory)
     await writeFile(join(directory, 'image.bin'), Buffer.from([0, 1, 2]))
     await writeFile(join(directory, 'invalid.txt'), Buffer.from([0xff]))
-    await writeFile(
-      join(directory, 'large.txt'),
-      Buffer.alloc(256 * 1024 + 1, 97)
-    )
 
     await expect(
       readWorkspaceFile(directory, '../outside.txt')
@@ -190,8 +191,33 @@ describe('workspace file browsing', () => {
     await expect(
       readWorkspaceFile(directory, 'invalid.txt')
     ).rejects.toThrow('有效 UTF-8')
-    await expect(
-      readWorkspaceFile(directory, 'large.txt')
-    ).rejects.toThrow('超过 256KB')
+  })
+
+  it('reads large UTF-8 files page by page without a total preview limit', async () => {
+    const directory = await mkdtemp(join(tmpdir(), 'goodbuddy-files-'))
+    temporaryDirectories.push(directory)
+    const firstPage = 'a'.repeat(256 * 1024 - 1)
+    const content = `${firstPage}你\n第二页`
+    await writeFile(join(directory, 'large.txt'), content)
+
+    const first = await readWorkspaceFile(directory, 'large.txt')
+    expect(first).toMatchObject({
+      content: firstPage,
+      offsetBytes: 0,
+      nextOffsetBytes: 256 * 1024 - 1,
+      truncated: true
+    })
+
+    const second = await readWorkspaceFile(
+      directory,
+      'large.txt',
+      first.nextOffsetBytes
+    )
+    expect(second).toMatchObject({
+      content: '你\n第二页',
+      offsetBytes: first.nextOffsetBytes,
+      truncated: false
+    })
+    expect(`${first.content}${second.content}`).toBe(content)
   })
 })