fix: restore manual context compaction for direct models

Local direct-model text conversations could not use manual context compaction,
and disabling automatic compression prevented saved summaries from being reused.
Restore the composer action regardless of the automatic setting or threshold,
use the selected summary connection, and persist summaries for subsequent
requests while retaining recent turns and full conversation history.

Image connections, remote channel conversations, and SSH projects remain outside
the local direct-model compaction workflow. Add focused regressions, a local
Electron end-to-end test, and an opt-in two-request live probe.

Validation: staged snapshot passed typecheck, changed-file ESLint, 13 focused
tests, and the Electron end-to-end test with two local HTTP fixture requests.
No additional real-model calls were made during commit verification.

Release note: 修复本地直连文本模型无法手动压缩上下文的问题。现在关闭自动压缩或尚未达到触发阈值时，也可点击“压缩上下文”；摘要会随会话保存并用于后续对话，同时保留最近轮次和完整历史。

docs/features/direct-model-agent/ui-design.md

@@ -21,6 +21,11 @@
 
 Job、Subjob 和 Run 继续是内部对象，不创建树形 UI、独立路由或操作菜单。
 
+本地直连文本会话复用输入区已有的“压缩上下文”按钮，不依赖原生 Runtime 快照，
+也不因自动压缩关闭或上下文低于触发阈值而隐藏。运行中或压缩中按钮禁用；图像连接、
+远程通道会话和 SSH 项目不显示直连模型压缩入口。完成后沿用应用通知及压缩后上下文估算，
+摘要随会话保存。压缩行为见[技术设计](./technical-design.md#101-本地直连手动压缩)。
+
 ## 2. 设置中的能力目录
 
 直连模型工具目录使用两个本机开发分组：