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>

CHANGELOG.md

@@ -2,6 +2,50 @@
 
 All notable changes to Switchboard Core are documented here.
 
+## v0.6.4 — Admin Health/Metrics Tab + Cluster Merge
+
+Structural consolidation: cluster dashboard merges into Admin as a Health tab.
+Comprehensive metrics endpoint for runtime, DB, cluster, and extension stats.
+
+### Added
+
+- **Admin Health tab**: New "Health" section under Monitoring in the Admin
+  surface. Auto-refreshing panels for runtime, database, cluster, and extension
+  metrics with configurable poll interval (5/10/30/60s).
+- **`GET /api/v1/admin/metrics`**: Single JSON endpoint returning all platform
+  metrics — runtime (goroutines, heap, GC, uptime, FDs), DB pool (latency,
+  active/idle/max, PG-only dead tuples + active backends), cluster nodes
+  (when PG multi-node), and extension stats (Starlark exec/errors/duration,
+  trigger fires, event bus publish/deliver counts).
+- **Fattened heartbeat payload**: Cluster heartbeat JSONB now carries stack
+  usage, GC CPU%, extensions loaded, Starlark execution counters, and trigger
+  fire count — visible in per-node cluster cards.
+- **Sandbox execution tracking**: `ExecPackage` and `CallEntryPoint` now
+  increment atomic counters (exec, errors, duration) and the previously
+  declared but unused `SandboxExecutionsTotal` Prometheus counter.
+- **Event bus counters**: `Bus.PublishCount()` and `Bus.DeliverCount()` track
+  cumulative publish/deliver operations.
+- **Trigger fire counter**: `Engine.FireCount()` tracks cumulative trigger
+  fires across webhook, event, and scheduled triggers.
+- 4 new handler tests (metrics SQLite shape, cluster shape, extension counters).
+- 1 new cluster test (fattened heartbeat payload).
+
+### Changed
+
+- **Health endpoint consolidation**: `/health` and `/api/v1/health` now use a
+  shared `buildHealthResponse()` function returning identical JSON. Both now
+  include `registration_enabled` when the database is connected.
+- **Block renderer `requires` removed**: `mermaid-renderer`, `katex-renderer`,
+  `csv-table`, and `diff-viewer` no longer require `["chat"]` — they activate
+  on any surface (notes, docs, etc.).
+
+### Removed
+
+- **`cluster-dashboard` package**: Standalone surface package retired. Cluster
+  node visibility is now in Admin > Monitoring > Health.
+
+---
+
 ## v0.6.3 — Dead Code Sweep + Registry Fix
 
 Pre-fork hardening: fix broken registry install, add registry settings UI,