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.go

@@ -0,0 +1,56 @@
+package eventbus
+
+import "sync"
+
+type RingBuffer[T any] struct {
+	mu    sync.Mutex
+	buf   []T
+	head  int
+	tail  int
+	count int
+	cap   int
+}
+
+func NewRingBuffer[T any](capacity int) *RingBuffer[T] {
+	return &RingBuffer[T]{
+		buf: make([]T, capacity),
+		cap: capacity,
+	}
+}
+
+func (r *RingBuffer[T]) Push(item T) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	r.buf[r.head] = item
+	r.head = (r.head + 1) % r.cap
+
+	if r.count == r.cap {
+		r.tail = (r.tail + 1) % r.cap
+	} else {
+		r.count++
+	}
+}
+
+func (r *RingBuffer[T]) Pop() (T, bool) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	var zero T
+	if r.count == 0 {
+		return zero, false
+	}
+
+	item := r.buf[r.tail]
+	r.buf[r.tail] = zero
+	r.tail = (r.tail + 1) % r.cap
+	r.count--
+
+	return item, true
+}
+
+func (r *RingBuffer[T]) Len() int {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return r.count
+}