[Feature] Alert channel implementations (Teams, Slack, GitHub, Email)

[adamgell]

Summary

Implement alert channel dispatchers for the ic alert command. The CLI command scaffolding (AlertCommand.cs) and drift detection core (DriftDetectionService, ExportNormalizer) already exist.

Channels

• ic alert teams --webhook <url> --report drift-report.json — Post adaptive card to Teams webhook
• ic alert slack --webhook <url> --report drift-report.json — Post block kit message to Slack webhook
• ic alert github --repo owner/repo --token X --report drift-report.json — Open GitHub issue with markdown report
• ic alert email --to ops@contoso.com --report drift-report.json — Send email via SMTP

Notes

• Each channel should format the drift report appropriately for the platform
• Severity-based coloring/icons where supported
• Exit code 0 on success, 1 on delivery failure

Follows from #115 (drift detection core — closed).