Add helper tools for common PromQL patterns (rate, irate, sum by, topk)

**ID:** tool.query_macros
**Priority:** P2
**Component:** mcp/tools

Provide ergonomic wrappers that construct PromQL based on parameters and call query/_range under the hood.

### Acceptance Criteria
- Tools: promql_rate, promql_topk, promql_sum_by
- Return both constructed query and result
- Validation for required params with helpful errors
- Tests for macro correctness

**Estimate:** 3
