feat: add MCP healthcheck probes and new templates (grafana, home-assistant, node-red)

- Add healthCheck spec to templates and servers (tool, arguments, interval, timeout, failureThreshold)
- Add healthStatus, lastHealthCheck, events fields to instances
- Create grafana, home-assistant, node-red templates with healthcheck probes
- Add healthcheck probes to existing templates (github, slack, postgres, jira)
- Show HEALTH column in `get instances` and Events section in `describe instance`
- Display healthCheck details in `describe server` and `describe template`
- Schema + storage + display only; actual probe runner is future work

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/slack.yaml

@@ -4,6 +4,9 @@ description: Slack MCP server for reading channels, messages, and user info
 packageName: "@anthropic/slack-mcp"
 transport: STDIO
 repositoryUrl: https://github.com/modelcontextprotocol/servers/tree/main/src/slack
+healthCheck:
+  tool: list_channels
+  arguments: {}
 env:
   - name: SLACK_BOT_TOKEN
     description: Slack bot token (xoxb-...)