Alertmanager integration: list/create silences and list alerts

[brenoepics]

ID: alertmanager.integration
Priority: P1
Component: new module: alertmanager_{client,repository,tools}

Add ALERTMANAGER_URL config and tools: alertmanager_list_silences, alertmanager_create_silence, alertmanager_list_alerts.

Acceptance Criteria

• Config via env (URL, auth, TLS reuse)
• Tools expose minimal schemas for silence matchers and durations
• Unit tests parsing fixture responses
• Graceful errors when AM unavailable

Estimate: 8