Feat v0.6.4 admin health/metrics tab + cluster merge

Admin Health tab under Monitoring with auto-refreshing runtime, DB,
cluster, and extension metrics panels. New GET /api/v1/admin/metrics
endpoint. Fattened heartbeat JSONB with stack, GC CPU%, extension
count, sandbox stats, trigger fires. Sandbox runner and trigger engine
now track cumulative execution counters. Event bus tracks publish/deliver
counts. Health endpoints consolidated via shared builder. Block renderers
(mermaid, katex, csv-table, diff-viewer) no longer require chat.
cluster-dashboard package retired — merged into admin Health tab.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/triggers/engine.go

@@ -9,6 +9,7 @@ import (
 	"encoding/json"
 	"log"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/robfig/cron/v3"
@@ -30,10 +31,15 @@ type Engine struct {
 	unsubs  map[string]func()        // trigger_id → bus unsubscribe
 	cronIDs map[string]cron.EntryID  // scheduled_task_id → cron entry
 
+	fireCount atomic.Int64 // cumulative trigger fires
+
 	ctx    context.Context
 	cancel context.CancelFunc
 }
 
+// FireCount returns the cumulative number of trigger fires.
+func (e *Engine) FireCount() int64 { return e.fireCount.Load() }
+
 // New creates a trigger engine. Call Start() to begin processing.
 func New(stores store.Stores, runner *sandbox.Runner, bus *events.Bus) *Engine {
 	return &Engine{
@@ -201,8 +207,9 @@ func (e *Engine) logExecution(triggerID, scheduledTaskID string, firedAt time.Ti
 	}
 }
 
-// publishEvent emits a trigger lifecycle event on the bus.
+// publishEvent emits a trigger lifecycle event on the bus and increments the fire counter.
 func (e *Engine) publishEvent(label string, triggerID string) {
+	e.fireCount.Add(1)
 	if e.bus != nil {
 		payload, _ := json.Marshal(map[string]any{"trigger_id": triggerID})
 		e.bus.Publish(events.Event{