v4 architecture: pipeline stages, SDK, event bus, LLM-driven memory consolidation

- SDK PluginAPI (internal/plugin/sdk/): RegisterTool/RegisterStage/Subscribe/Publish
- EventBus (internal/events/): system-level pub/sub with wildcard support
- StageHost (internal/agent/core/stages.go): 7-stage message pipeline
- Agent core: on_input/pre_action/post_action/before_toolcall/after_toolcall/before_output/after_output
- Plugin Registry: SDK plugin registration and tool routing
- GraphDB.MergeEntities: entity consolidation with relation redirection
- memory_merge tool: allows LLM to merge similar entities
- Consolidation task: heartbeat detects conflicts, enqueues via IO for LLM decision
- _consolidation_ internal channel for system-level memory maintenance
- Comprehensive documentation: ARCHITECTURE.md, PLAN.md, DESIGN.md, README.md
- 54 tests across all packages, all passing

internal/events/bus.go

@@ -0,0 +1,69 @@
+package events
+
+import (
+	"fmt"
+	"sync"
+)
+
+type EventType string
+
+const (
+	EventRawInput    EventType = "raw_input"
+	EventAgentOutput EventType = "agent_output"
+	EventToolCall    EventType = "tool_call"
+	EventReasoning   EventType = "reasoning"
+	EventSystem      EventType = "system"
+	EventAll         EventType = "*"
+)
+
+type Event struct {
+	Type      EventType              `json:"type"`
+	Source    string                 `json:"source"`
+	Payload   map[string]interface{} `json:"payload"`
+	Timestamp int64                  `json:"timestamp"`
+}
+
+type Handler func(event *Event)
+
+type Bus struct {
+	mu   sync.RWMutex
+	subs map[EventType][]Handler
+}
+
+func NewBus() *Bus {
+	return &Bus{
+		subs: make(map[EventType][]Handler),
+	}
+}
+
+func (b *Bus) Publish(evt *Event) {
+	b.mu.RLock()
+	allHandlers := b.subs[EventAll]
+	typeHandlers := b.subs[evt.Type]
+	b.mu.RUnlock()
+
+	for _, h := range allHandlers {
+		h(evt)
+	}
+	for _, h := range typeHandlers {
+		h(evt)
+	}
+}
+
+func (b *Bus) Subscribe(eventType EventType, handler Handler) func() {
+	b.mu.Lock()
+	b.subs[eventType] = append(b.subs[eventType], handler)
+	b.mu.Unlock()
+
+	return func() {
+		b.mu.Lock()
+		defer b.mu.Unlock()
+		list := b.subs[eventType]
+		for i, h := range list {
+			if fmt.Sprintf("%p", h) == fmt.Sprintf("%p", handler) {
+				b.subs[eventType] = append(list[:i], list[i+1:]...)
+				break
+			}
+		}
+	}
+}