fix: release runtime status probes after checks

Chat startup and Runtime switching previously cached status-only Runtime instances, leaving managed OpenCode servers running without active conversations. Status checks now resolve the selected or configured default Runtime through a disposable probe and release it immediately.

Project execution Runtimes remain cached by model selection and execution space, preserving parallel work across different projects.

Release note: 修复启动或切换 Runtime 后空闲 OpenCode 服务持续驻留的问题；状态检查完成后会立即回收，多个项目仍可并行执行。

src/main/ipc.test.ts

@@ -6277,6 +6277,23 @@ describe('registerIpcHandlers agent terminal state', () => {
     expect(selectedRuntimes.getStatus).toHaveBeenCalledWith(
       secondSelection
     )
+    harness.getResolvedSettings.mockResolvedValueOnce({
+      provider: 'opencode',
+      defaultModelProfileId: firstProfileId,
+      opencodeBaseUrl: '',
+      opencodeEmbedded: true,
+      opencodeModelSource: { kind: 'platform' },
+      continueModelSource: { kind: 'platform' }
+    })
+    await expect(
+      harness.statusHandler?.(event)
+    ).resolves.toEqual(
+      expect.objectContaining({ label: 'model-two' })
+    )
+    expect(selectedRuntimes.getStatus).toHaveBeenLastCalledWith({
+      provider: 'opencode'
+    })
+    expect(fallbackRuntime.getStatus).not.toHaveBeenCalled()
 
     await Promise.all([
       harness.handler?.(event, {