Engine Decomposition: Clean core — eliminate BuildFromConfig switch & add workflow dependencies (Phase 4) #84

Overview

The final phase: eliminate the BuildFromConfig switch statement entirely, remove all post-init wiring from engine.go, and add workflow dependency validation.

Reference: docs/PLAN_ENGINE_DECOMPOSITION.md — Phase 4
Depends on: Phase 3 (#83)

Tasks

Remove Switch Statement

• Replace entire switch modCfg.Type { block with plugin factory map lookup:

  factory, exists := e.moduleFactories[modCfg.Type]
      return fmt.Errorf("unknown module type %q: no plugin provides this type", modCfg.Type)
  }
  mod = factory(modCfg.Name, modCfg.Config)

Remove Post-Init Wiring

• Replace all post-init wiring logic with wiring hook execution loop
• Remove canHandleTrigger() function — use trigger registry

Remove Direct Imports

• engine.go no longer imports module/, handlers/, or modular cache/database/reverseproxy/scheduler
• Core imports only: modular, config, schema, capability, secrets, plugin

Workflow Dependency Validation

• Add requires section parsing to BuildFromConfig (validates before building)
• Update all 37+ example configs with requires sections
• Auto-detection tool infers capabilities from module types when requires is omitted

Testing

• Structural negative tests: verify engine.go has no forbidden imports
• Workflow dependency tests: configs with missing capabilities produce clear errors
• Full regression suite passes

Documentation

• Update CLAUDE.md with new plugin-based architecture
• Create docs/PLUGIN_DEVELOPMENT.md — guide for creating new plugins

Success Criteria (from docs)

1. engine.go contains no module type switch statement
2. engine.go does not import module/ or handlers/
3. All negative tests pass (core rejects unknown types)
4. All positive tests pass (plugins provide types)
5. All 37+ example configs load successfully
6. A new plugin can be created without modifying any core file
7. Two plugins providing the same capability can be swapped without config changes
8. Core engine.go reduced to ~400 lines (from ~1647)

Post-Decomposition Core

StdEngine retains only: YAML parsing, module lifecycle, service registry, workflow dispatch, pipeline execution, trigger dispatch, and plugin loading.