feat(api): status ring buffer (last 100) + GET /status/recent endpoint

Bus.Publish now also appends each event to a per-topic ring buffer of
size 100. Bus.Recent(topic, n) returns the tail. New endpoint:

  GET /status/recent?n=N    → JSON array of last N status-diff events

This lets a fresh client (agents_dashboard launching cold) populate its
Status Feed panel with historical activity before subscribing to
/sse/status for live updates. Until now, new SSE subscribers only saw
events emitted AFTER they connected — making the panel useless for
recent history review.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/pubsub.go

@@ -14,14 +14,36 @@ type Event = any
 
 // Bus is a simple in-memory pub/sub hub.
 // Topics are arbitrary strings (e.g. "status", "logs/agent-id").
+// Per-topic ring buffer of recent events (default 100) lets new subscribers
+// or GET endpoints fetch the recent history.
 type Bus struct {
-	mu   sync.RWMutex
-	subs map[string][]chan Event
+	mu      sync.RWMutex
+	subs    map[string][]chan Event
+	recent  map[string][]Event
+	histCap int
 }
 
-// NewBus creates an initialised Bus.
+// NewBus creates an initialised Bus with a 100-event history per topic.
 func NewBus() *Bus {
-	return &Bus{subs: make(map[string][]chan Event)}
+	return &Bus{
+		subs:    make(map[string][]chan Event),
+		recent:  make(map[string][]Event),
+		histCap: 100,
+	}
+}
+
+// Recent returns up to n most recent events for topic (oldest first).
+// n <= 0 returns the whole buffer (up to histCap).
+func (b *Bus) Recent(topic string, n int) []Event {
+	b.mu.RLock()
+	defer b.mu.RUnlock()
+	buf := b.recent[topic]
+	if n <= 0 || n > len(buf) {
+		n = len(buf)
+	}
+	out := make([]Event, n)
+	copy(out, buf[len(buf)-n:])
+	return out
 }
 
 // Subscribe returns a channel that receives events published to topic.
@@ -48,12 +70,19 @@ func (b *Bus) Unsubscribe(topic string, ch <-chan Event) {
 	}
 }
 
-// Publish sends ev to all subscribers of topic.
-// Non-blocking: if a subscriber channel is full, the event is dropped for that subscriber.
+// Publish sends ev to all subscribers of topic and appends to ring history.
+// Non-blocking: if a subscriber channel is full, the event is dropped for that
+// subscriber. History is always retained (capped at histCap).
 func (b *Bus) Publish(topic string, ev Event) {
-	b.mu.RLock()
-	list := b.subs[topic]
-	b.mu.RUnlock()
+	b.mu.Lock()
+	buf := b.recent[topic]
+	buf = append(buf, ev)
+	if len(buf) > b.histCap {
+		buf = buf[len(buf)-b.histCap:]
+	}
+	b.recent[topic] = buf
+	list := append([]chan Event(nil), b.subs[topic]...)
+	b.mu.Unlock()
 	for _, ch := range list {
 		select {
 		case ch <- ev: