Support multiple Prometheus backends with selection parameter #23
Description
ID: multi.prometheus
Priority: P2
Component: repository + tools
Allow configuring multiple named Prometheus endpoints and pass a 'target' param to tools to select which one to use.
Acceptance Criteria
- Config: PROMETHEUS_TARGETS=json map of name->URL+auth
- tools/list schemas accept optional 'target'
- Default target remains current behavior
- Unit tests for target selection
Estimate: 8