feat: add lab-monitoring MCP server for Prometheus and Alertmanager

New MCP server that queries live Prometheus and Alertmanager HTTP APIs
with 8 tools: list_alerts, get_alert, search_metrics, get_metric_metadata,
query (PromQL), list_targets, list_silences, and create_silence.

Extends the MCP core with ModeCustom and NewGenericServer for servers
that don't require a database. Includes CLI with direct commands
(alerts, query, targets, metrics), NixOS module, and comprehensive
httptest-based tests.

Bumps existing binaries to 0.2.1 due to shared internal/mcp change.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

TODO.md

@@ -15,6 +15,7 @@
 ## New MCP Servers
 
 - [x] `nixpkgs-packages` - Index and search nixpkgs packages (implemented in `nixpkgs-search packages`)
+- [x] `lab-monitoring` - Query Prometheus and Alertmanager APIs (8 tools, no database required)
 
 ## Nice to Have