feat(syslog): list patterns and add a ready-made one through MCP

Patterns could only be added on the Syslog page, so an agent asked to map
them on the NASes couldn't. list_syslog_patterns gives each pattern's metric
and the ready-made ones not added; add_syslog_preset (admin, audited) adds one.
The key-picking helper moves beside the patterns for both callers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/plugins/syslog.md

@@ -26,7 +26,7 @@ puts them beside Steward's other readings.
   by the anomaly's host or SNMP device. Each anomaly row has "Syslog around
   it". Host metrics charts and SNMP device charts mark each error with a
   dashed red line.
-- **MCP.** `list_syslog_sources` and `search_syslog`.
+- **MCP.** `list_syslog_sources`, `search_syslog`, `list_syslog_patterns` and `add_syslog_preset`.
 
 ## Deploying