Engine Decomposition: Extract remaining 13 plugins (Phase 3)

[intel352]

Overview

Extract all remaining module types from engine.go into plugins, following the pattern established by the HTTP and Observability extractions in Phase 2.

Reference: docs/PLAN_ENGINE_DECOMPOSITION.md — Phase 3
Depends on: Phase 2 (#82)

Extraction Order (by dependency and risk)

Order	Plugin	Module Types	Steps	Handler	Trigger
3a	workflow-plugin-messaging	messaging.broker, messaging.broker.eventbus, messaging.handler, messaging.nats, messaging.kafka + 2	step.publish	MessagingWorkflowHandler	event, eventbus
3b	workflow-plugin-statemachine	statemachine.engine, state.tracker, state.connector	—	StateMachineWorkflowHandler	—
3c	workflow-plugin-auth	auth.jwt, auth.user-store	—	— (wiring hook)	—
3d	workflow-plugin-storage	storage.s3, storage.local, storage.gcs, storage.sqlite, persistence.store, database.workflow	step.db_query, step.db_exec	—	—
3e	workflow-plugin-api	api.handler, api.query, api.command, api.gateway, openapi.consumer + 2	—	—	—
3f	workflow-plugin-pipeline-steps	—	step.validate, step.transform, step.set, step.conditional, step.shell_exec, step.deploy, step.scan_*	—	—
3g	workflow-plugin-cicd	—	12 CI/CD step types	—	—
3h	workflow-plugin-featureflags	featureflag.service	step.feature_check, step.feature_set	—	—
3i	workflow-plugin-secrets	secrets.vault, secrets.aws	—	—	—
3j	workflow-plugin-modular-compat	scheduler.modular, cache.modular, database.modular	—	—	—
3k	workflow-plugin-scheduler	—	—	SchedulerWorkflowHandler	schedule
3l	workflow-plugin-integration	—	—	IntegrationWorkflowHandler	—
3m	workflow-plugin-ai	dynamic.component	AI step types (4)	—	—

Per-Plugin Pattern

1. Create plugin package under plugins/<name>/
2. Implement EnginePlugin interface
3. Move factory logic from engine.go switch cases
4. Move step registrations from main.go
5. Remove switch case from engine.go
6. Write negative test (core rejects type without plugin)
7. Write positive test (plugin provides type correctly)
8. Update main.go to load plugin

Sub-Issues (Optional)

This can be broken into individual plugin PRs if desired — each plugin extraction is independent once Phase 2 establishes the pattern.

[intel352]

Phase 3 is already fully complete. All 13 plugins have been extracted into the plugins/ directory. The engine.go BuildFromConfig no longer has hardcoded module type factories — all module types are registered via the plugin system through LoadPlugin(). This includes: messaging, statemachine, auth, storage, api, pipelinesteps, cicd, featureflags, secrets, modularcompat, scheduler, integration, and ai plugins.