Add severity-based routing for escalations with config-driven targets.
Changes:
- EscalationConfig type with severity routes and external channels
- beads/beads_escalation.go: Escalation bead operations (create/ack/close/list)
- Refactored gt escalate command with subcommands:
- list: Show open escalations
- ack: Acknowledge an escalation
- close: Resolve with reason
- stale: Find unacknowledged escalations past threshold
- show: Display escalation details
- Added TypeEscalationAcked and TypeEscalationClosed event types
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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