fix(hub): re-dial SSH-pull agents when a connection silently dies (#2126)

The per-system updater reused an SSH client across ticks and ran the data
exchange with no deadline. If a connection went half-open (dead peer that
never sends RST/FIN) or an agent accepted the session but never wrote a
response, the read in fetchDataViaSSH blocked forever. Because
StartUpdater calls update() synchronously on its ticker, a blocked read
froze the whole per-system goroutine: the ticker's subsequent ticks were
dropped, no error was returned so the system stayed "up", and the agent
was never re-dialed until the hub process restarted.

Bound each SSH data exchange with sshOperationTimeout via runWithTimeout:
on timeout the connection is torn down (unwinding the blocked read) and a
retryable error is returned, so the next tick re-dials. Also enable TCP
keep-alive on dialed connections as a backstop for genuine network death.

Fixes #2041

internal/hub/systems/ssh_timeout_test.go

@@ -0,0 +1,56 @@
+//go:build testing
+
+package systems
+
+import (
+	"errors"
+	"testing"
+	"testing/synctest"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestRunWithTimeout covers the guard added for issue #2041: the per-system SSH
+// data exchange must never block the updater indefinitely on a dead connection.
+func TestRunWithTimeout(t *testing.T) {
+	t.Run("returns the operation result when it completes before the timeout", func(t *testing.T) {
+		synctest.Test(t, func(t *testing.T) {
+			wantErr := errors.New("boom")
+			onTimeoutCalled := false
+
+			retry, err := runWithTimeout(10*time.Second, func() (bool, error) {
+				return true, wantErr
+			}, func() { onTimeoutCalled = true })
+
+			assert.True(t, retry, "should return the operation's retry value")
+			assert.Equal(t, wantErr, err, "should return the operation's error")
+			assert.False(t, onTimeoutCalled, "onTimeout must not fire when the op completes")
+		})
+	})
+
+	t.Run("times out and tears down the connection when the op blocks", func(t *testing.T) {
+		synctest.Test(t, func(t *testing.T) {
+			// unblock simulates a half-open connection: the op is stuck reading a
+			// response that never arrives until the connection is torn down.
+			unblock := make(chan struct{})
+			onTimeoutCalled := false
+			start := time.Now()
+
+			retry, err := runWithTimeout(5*time.Second, func() (bool, error) {
+				<-unblock
+				return false, nil
+			}, func() {
+				onTimeoutCalled = true
+				close(unblock) // tearing down the connection releases the blocked read
+			})
+
+			assert.Equal(t, 5*time.Second, time.Since(start), "should return exactly at the timeout")
+			assert.True(t, retry, "a timeout should be retryable so the next tick re-dials")
+			assert.Error(t, err, "a timeout must surface an error so the system is set down")
+			assert.True(t, onTimeoutCalled, "onTimeout must fire so the dead connection is closed")
+
+			synctest.Wait() // ensure the released op goroutine exits cleanly
+		})
+	})
+}