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/handlers/admin_metrics.go

@@ -0,0 +1,24 @@
+package handlers
+
+import (
+	"github.com/gin-gonic/gin"
+
+	"switchboard-core/metrics"
+)
+
+// MetricsHandler serves the admin metrics endpoint.
+type MetricsHandler struct {
+	collector *metrics.Collector
+}
+
+// NewMetricsHandler creates a MetricsHandler.
+func NewMetricsHandler(c *metrics.Collector) *MetricsHandler {
+	return &MetricsHandler{collector: c}
+}
+
+// GetMetrics returns a full metrics snapshot.
+// GET /api/v1/admin/metrics
+func (h *MetricsHandler) GetMetrics(c *gin.Context) {
+	snap := h.collector.Collect(c.Request.Context())
+	c.JSON(200, snap)
+}