Add in-process event bus with ring buffer for workflow event broadcasting

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/claude-agent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/eventbus/ring_test.go

@@ -0,0 +1,109 @@
+package eventbus
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRingBuffer_PushPop(t *testing.T) {
+	ring := NewRingBuffer[int](5)
+
+	ring.Push(1)
+	ring.Push(2)
+	ring.Push(3)
+
+	v, ok := ring.Pop()
+	require.True(t, ok)
+	assert.Equal(t, 1, v)
+
+	v, ok = ring.Pop()
+	require.True(t, ok)
+	assert.Equal(t, 2, v)
+
+	v, ok = ring.Pop()
+	require.True(t, ok)
+	assert.Equal(t, 3, v)
+}
+
+func TestRingBuffer_Empty(t *testing.T) {
+	ring := NewRingBuffer[int](5)
+
+	v, ok := ring.Pop()
+	assert.False(t, ok)
+	assert.Equal(t, 0, v)
+}
+
+func TestRingBuffer_OverwriteOldest(t *testing.T) {
+	ring := NewRingBuffer[int](4)
+
+	ring.Push(1)
+	ring.Push(2)
+	ring.Push(3)
+	ring.Push(4)
+	ring.Push(5)
+	ring.Push(6)
+
+	var values []int
+	for {
+		v, ok := ring.Pop()
+		if !ok {
+			break
+		}
+		values = append(values, v)
+	}
+
+	assert.Equal(t, []int{3, 4, 5, 6}, values)
+}
+
+func TestRingBuffer_Len(t *testing.T) {
+	ring := NewRingBuffer[int](5)
+
+	assert.Equal(t, 0, ring.Len())
+
+	ring.Push(1)
+	assert.Equal(t, 1, ring.Len())
+
+	ring.Push(2)
+	ring.Push(3)
+	assert.Equal(t, 3, ring.Len())
+
+	ring.Pop()
+	assert.Equal(t, 2, ring.Len())
+
+	ring.Push(4)
+	ring.Push(5)
+	ring.Push(6)
+	ring.Push(7)
+	assert.Equal(t, 5, ring.Len())
+
+	ring.Push(8)
+	assert.Equal(t, 5, ring.Len())
+}
+
+func TestRingBuffer_Concurrent(t *testing.T) {
+	ring := NewRingBuffer[int](100)
+	var wg sync.WaitGroup
+
+	for i := 0; i < 10; i++ {
+		wg.Add(1)
+		go func(id int) {
+			defer wg.Done()
+			for j := 0; j < 100; j++ {
+				ring.Push(id*100 + j)
+			}
+		}(i)
+	}
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for i := 0; i < 500; i++ {
+			ring.Pop()
+		}
+	}()
+
+	wg.Wait()
+}