Error Recovery: Additional notification handlers (Slack/webhook) + criteria-rich dispatch

[RyanTheRobothead]

Builds on the EventHandler plugin system introduced by PR #312. Adds new AbstractEventHandler implementations alongside the existing NotificationHandler:

• SlackNotificationHandler (incoming webhook)
• WebhookNotificationHandler (generic HTTP POST)

Also extends the trigger criteria beyond NotificationHandlerSettings' single alert_level threshold:

• Per-event_type matching (e.g., always alert on NODE_LOST even at INFO)
• Per-source filtering (specific nodes/managers/workflows)
• Rate-limiting + deduplication per (channel, event_type) pair to prevent alert storms
• Optional digesting (batch N events into one notification within a window)

This is the multi-channel + criteria-rich layer on top of the foundation that PR #312 lands. Re-scopes #122 rather than replacing it.

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Roadmap metadata

• Phase: 1.0-rc — Hardening
• Project: Advanced Error Recovery
• Blocked by: Feature error handling MVP #312, Event Manager: Async/background EventHandler dispatch #338
• Blocks: Error Recovery: Hanging-action detection + alerting integration #324, Error Recovery: Re-decide Event-Manager-plugins vs dedicated Error Manager #325

Roadmap note: Depends on #338 — outbound HTTP to Slack/PagerDuty/webhooks cannot run inline in log_event without slowing ingestion.

Generated as part of the MADSci → 1.0 roadmap. See docs/roadmap.md for context.