feat(alerts): add Slack channel with webhooks and Block Kit - Issue #40 (44 tests)

Implements Slack integration for alert delivery:
- SlackMessageStyle enum (simple, blocks, attachment)
- SlackConfig for webhook URL, channel, username, icon
- SlackMessageResult for delivery tracking
- SlackMessageFormatter with Block Kit support
- SlackChannel implementing AlertChannel protocol
- create_slack_channel factory function

Features:
- Three formatting styles (simple text, blocks, attachments)
- Priority-based colors and emojis
- Category emojis for visual identification
- @mention support for critical alerts
- Timestamp and source inclusion options
- Data field display in messages
- Webhook URL validation (hooks.slack.com)
- Retry logic with exponential backoff
- Rate limit handling (429 status with Retry-After)
- Server error retry (500+) vs client error fail-fast (400)
- Latency tracking for performance monitoring
- Sync and async delivery methods
- Test webhook functionality

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>