feat: add option to alert for stale GitHub security alerts

[sgtoj]

Summary

• Add scheduled security alert monitoring that scans org-wide Dependabot, code scanning, and secret scanning alerts, filtering by configurable minimum age and severity thresholds
• Introduce Slack notification formatting for security alert reports with per-repo breakdowns
• Add new configuration options (APP_GH_SEC_ALERTS_ENABLED, APP_GH_SEC_ALERTS_MIN_AGE_DAYS, APP_GH_SEC_ALERTS_MIN_SEVERITY) and a scheduled handler endpoint

Changes

• internal/github/client/security_alerts.go - New GitHub client methods to fetch and filter Dependabot, code scanning, and secret scanning alerts across the org
• internal/notifiers/slack_messages.go - Slack message formatting for security alert reports
• internal/app/handlers.go - New HandleSecurityAlerts handler wired into the scheduled events flow
• internal/config/config.go - Configuration parsing for security alert settings
• internal/domain/ - New SecurityAlert, SecurityAlertsReport, and SecurityAlertsConfig types plus SecurityAlertChecker interface
• Tests added for handler, config helpers, Slack formatting, and GitHub client
• Updated docs (README, Lambda README, setup guides) and GitHub App manifest with new permissions