Summary

Implement the ExternalTriggerStrategy for the pluggable ceremony scheduling system.

Ceremonies fire on external signals: webhooks, CI/CD events, git events, MCP tool invocations. Bridges the synthetic org with real-world development workflows.

Design Reference

See Ceremony Scheduling design page -- "External-Trigger" strategy section.

Key Behaviors

• should_fire_ceremony: checks if a matching external event has been received (via context.external_events)
• should_transition_sprint: returns target status on configured external signal (e.g., deploy_complete)
• on_sprint_activated: registers external event listeners/subscriptions
• on_sprint_deactivated: unregisters listeners
• on_external_event: receives and buffers external signals for evaluation
• Default velocity calculator: PointsPerSprintVelocityCalculator

Strategy Config

strategy_config:
  sources:
    - type: webhook
      endpoint: /hooks/ceremony
    - type: git_event
      events: [push, tag]

Per-ceremony config:

policy_override:
  strategy_config:
    on_external: pr_merged

Implementation

• New: src/synthorg/engine/workflow/strategies/external_trigger.py
• New: tests/unit/engine/workflow/strategies/test_external_trigger.py
• Implement CeremonySchedulingStrategy protocol from feat: sprint ceremony runtime scheduler with task-driven cadence support #961
• Stateful strategy (uses lifecycle hooks for event subscriptions)

Integration Points

• MCP bridge (src/synthorg/tools/) for MCP tool invocation events
• API layer for webhook endpoint registration
• CeremonyEvalContext.external_events tuple

Dependencies

• feat: sprint ceremony runtime scheduler with task-driven cadence support #961 (protocol interfaces + reference implementation)
• feat: event-driven ceremony scheduling strategy #971 (EventDrivenStrategy -- shares event subscription patterns)