MCP: allow filtering list_alert_rules by cluster

[ebarron]

Summary

Add an optional cluster parameter to the list_alert_rules MCP tool so users can list only alert rules that target a specific cluster (or set of clusters), instead of always returning every rule defined in alert_rules.yml / ems_alert_rules.yml.

Background

list_alert_rules reads the rule files via the rule manager and returns the full inventory. Cluster scoping today is only possible if a cluster name happens to appear inside a rule's expr (e.g. volume_size_used_percent{cluster="prod-east-1"} > 90), and even then the user has to grep the response themselves.

In multi-cluster Harvest deployments operators commonly want to answer:

• "Which alert rules apply to prod-east-1?"
• "Show me all rules that mention lab-cluster-2."

…without paging through dozens of unrelated rules.

Proposed enhancement

Add an optional argument:

• cluster — exact cluster name, or
• cluster_match — regex pattern.

When provided, filter the returned AlertRules and EMSRules lists to only include rules whose:

• expr contains a label matcher referencing the cluster (cluster="X", cluster=~"pattern", or substring match), or
• labels.cluster matches the requested cluster(s).

When omitted, behavior is unchanged (return everything).

Use cases

• "What alerts are configured for my prod-east-1 cluster?"
• Per-datacenter rule audits.
• Chatbot conversations scoped to a single cluster ("show me my volume rules for cluster X").

Notes

Filed as part of a NAbox chatbot integration review. Companion issues: #4286 (infrastructure_health), #4287 (get_active_alerts). Together these three are the only harvest-mcp tools that currently can't be scoped to a cluster — the rest already allow it via PromQL label matchers in their query arguments.