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/workspace-file-access.ts

@@ -67,6 +67,84 @@ export async function readBoundedUtf8File(
   }
 }
 
+export async function readUtf8FileRange(
+  filePath: string,
+  offsetBytes: number,
+  maximumBytes: number,
+  invalidUtf8Message: string
+): Promise<{
+  content: string
+  size: number
+  offsetBytes: number
+  bytesRead: number
+  truncated: boolean
+}> {
+  const handle = await open(filePath, 'r')
+  try {
+    const metadata = await handle.stat()
+    if (
+      !metadata.isFile() ||
+      !Number.isSafeInteger(offsetBytes) ||
+      offsetBytes < 0 ||
+      offsetBytes > metadata.size
+    ) {
+      throw new Error('工作区读取范围无效')
+    }
+    const requestedBytes = Math.min(
+      maximumBytes,
+      metadata.size - offsetBytes
+    )
+    const data = Buffer.alloc(requestedBytes)
+    let bytesRead = 0
+    while (bytesRead < data.length) {
+      const result = await handle.read(
+        data,
+        bytesRead,
+        data.length - bytesRead,
+        offsetBytes + bytesRead
+      )
+      if (result.bytesRead === 0) {
+        break
+      }
+      bytesRead += result.bytesRead
+    }
+    const selected = data.subarray(0, bytesRead)
+    const reachesEnd = offsetBytes + bytesRead >= metadata.size
+    let decoded: string | undefined
+    let decodedBytes = bytesRead
+    for (
+      let trailingBytes = 0;
+      trailingBytes <= (reachesEnd ? 0 : 3);
+      trailingBytes += 1
+    ) {
+      decodedBytes = bytesRead - trailingBytes
+      if (decodedBytes < 0) {
+        break
+      }
+      try {
+        decoded = new TextDecoder('utf-8', { fatal: true }).decode(
+          selected.subarray(0, decodedBytes)
+        )
+        break
+      } catch {
+        // A page may end in the middle of one UTF-8 character.
+      }
+    }
+    if (decoded === undefined) {
+      throw new Error(invalidUtf8Message)
+    }
+    return {
+      content: decoded,
+      size: metadata.size,
+      offsetBytes,
+      bytesRead: decodedBytes,
+      truncated: offsetBytes + decodedBytes < metadata.size
+    }
+  } finally {
+    await handle.close()
+  }
+}
+
 export async function readBoundedFile(
   filePath: string,
   maximumBytes: number,