fix: stabilize Linux release validation

Ubuntu validation exposed a missing Unix socket test import, a platform-dependent cancellation-message assertion, and a maximum-frame test that could exceed the default timeout under shared-runner load. The tests now exercise the intended behavior consistently without changing product code.

src/agent-daemon/private-endpoint.test.ts

@@ -4,7 +4,11 @@ import {
   rmSync,
   unlinkSync
 } from 'node:fs'
-import { createConnection, type Socket } from 'node:net'
+import {
+  createConnection,
+  createServer,
+  type Socket
+} from 'node:net'
 import { tmpdir } from 'node:os'
 import { join, resolve } from 'node:path'
 import { PassThrough } from 'node:stream'

src/agent-daemon/workspace-registry.test.ts

@@ -243,7 +243,10 @@ describe('WorkspaceRegistry', () => {
     abort.abort(new Error('cancelled'))
     releaseInspection?.('not-a-repository')
 
-    await expect(pending).rejects.toThrow('cancelled')
+    await expect(pending).rejects.toMatchObject({
+      code: 'aborted',
+      message: 'Workspace operation was aborted'
+    })
     expect(registry.activeHandleCount()).toBe(0)
     expect(existsSync(storagePath)).toBe(false)
   })