chore(release): pin K-brain beta.6 for LiveAgent beta.11

[xiaozhou26]

Closes #964

Summary

• Pin backend and Computer Use helper downloads to K-brain v0.107.6-beta.6 (df192b6), including Fix AI release notes generation #25/Features #26.
• Prepare LiveAgent v2.0.0-beta.11 with fix(chat): 提问超时且没有答案时，卡片显示没有替你做选择 #957/perf(kbrain): 启动时不再重传已迁移的旧会话，存在性确认改为一次列表 #959/fix(kbrain): 删除的旧版供应商重启后不再被重新导入 #961/fix(settings): 供应商保存被 K-brain 拒绝后不再拖垮后续保存 #963.
• Update all four target URLs and SHA-256 values; no backend source build in LA CI.

Validation

• K-brain release CI passed and 13 assets verified against GitHub SHA-256 digests and SHA256SUMS.
• Downloaded macOS backend: isolated startup/health, large legacy import and readable HTTP 413, provider-specific metadata save/restart, downgrade startup with beta.5.
• Release scripts: 44 passed.
• Four-target package verification and full GUI suite against new backend in progress before tagging.

[xiaozhou26]

发版前验证完成：四目标 prepare + backend/helper packaging checksum 验证通过；新锁定 beta.6 后端下 GUI 3053 passed / 1 skipped；K-brain config/backend/routing/cmd/kn/tui 全部竞态测试通过。已下载 release 二进制运行隔离健康检查、>5 MiB 导入/64 MiB 超限、按供应商元数据保存重启和 beta.5 降级启动。等待 CI 后合并并打 beta.11 标签。