Curated prompts list that chain to tools (diagnostics, top-N, alerts summary)

[brenoepics]

ID: prompts.builtin_set
Priority: P1
Component: mcp/prompts + main router

Replace compat prompts with a small set that instruct clients to call new tools for diagnostics.

Acceptance Criteria

• Enable prompts/list and prompts/get in router (non-compat)
• Add prompts: diagnose_high_cpu, alert_summary, explore_metric
• Arguments validated and errors on missing args
• Snapshot tests for prompt payloads

Estimate: 2