test(e2e): Implement multi-repository event-driven testing

[dangazineu]

Create comprehensive E2E tests for multi-repository event-driven workflows, validating the complete orchestration system.

Description

This issue creates end-to-end tests that validate the complete event-driven multi-repository workflow orchestration system across multiple repositories.

Test Scenarios

• Fan-out/fan-in with event-driven architecture
• Subscription filter evaluation and schema compatibility
• Diamond dependency resolution
• Partial failure recovery and smart resume
• Long-running workflows with container persistence

Key Test: Event-Driven Fan-Out/Fan-In

# Parent (go-lib) emits library_built event
# Children (app-one, app-two) subscribe and emit service_updated events  
# Aggregator (release-bom) subscribes to both service_updated events
# Validates complete event flow and dependency coordination

Test Structure

1. Repository Setup: Create test fixture repositories with realistic configurations
2. Event Flow Validation: Verify events are emitted, filtered, and delivered correctly
3. Subscription Processing: Test CEL filter evaluation and schema compatibility
4. Execution Tree: Validate complete multi-repository execution trees
5. Failure Scenarios: Test partial failures and smart resume functionality

Acceptance Criteria

• Event-driven fan-out/fan-in scenario executes correctly
• Subscription filters evaluated properly with real repositories
• Schema compatibility validation works across repository boundaries
• Diamond dependencies resolved without conflicts
• Smart partial resume works for complex execution trees

Files to Modify

• internal/e2e/event_driven_test.go (new)
• internal/e2e/subscription_test.go (new)
• Test repository fixtures with realistic configurations

Related Issues

• Parent Epic: Execute multi-step workflows #21 - Execute multi-step workflows
• Design: Design(exec): A General-Purpose Workflow Engine for Multi-Repo Automation #98 - Event-driven workflow engine design
• Depends On: feat(engine): Implement containerized execution with security hardening #104-feat(steps): Implement semantic steps framework #107 (complete event-driven implementation)