Fix SetSampleRate not updating sample_rate dimension

The sample_rate common dimension was set once at service creation
time and never updated when SetSampleRate was called later. This
caused commands like 'gh skill publish' that override the sample
rate via PersistentPreRunE to report the wrong sample_rate in
telemetry events (e.g. 1 instead of 100).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/telemetry/telemetry.go

@@ -14,6 +14,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"slices"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -283,6 +284,7 @@ func (s *service) SetSampleRate(rate int) {
 	defer s.mu.Unlock()
 
 	s.sampleRate = rate
+	s.commonDimensions["sample_rate"] = strconv.Itoa(rate)
 }
 
 func (s *service) Flush() {

internal/telemetry/telemetry_test.go

@@ -579,6 +579,24 @@ func TestServiceSampling(t *testing.T) {
 		assert.False(t, called, "flusher should not be called after SetSampleRate reduced the rate")
 	})
 
+	t.Run("SetSampleRate updates sample_rate dimension", func(t *testing.T) {
+		t.Cleanup(stubDeviceID("test-device"))
+
+		var captured SendTelemetryPayload
+		svc := newService(func(p SendTelemetryPayload) { captured = p }, ghtelemetry.Dimensions{
+			"sample_rate": "1",
+		})
+		svc.sampleRate = 1
+		svc.sampleBucket = 0
+
+		svc.SetSampleRate(100)
+		svc.Record(ghtelemetry.Event{Type: "test"})
+		svc.Flush()
+
+		require.Len(t, captured.Events, 1)
+		assert.Equal(t, "100", captured.Events[0].Dimensions["sample_rate"])
+	})
+
 	t.Run("WithSampleRate option sets rate on construction", func(t *testing.T) {
 		t.Cleanup(stubDeviceID("test-device"))