API plugin factory creates concrete module types directly (plugins/api/plugin.go) #64

Problem

plugins/api/plugin.go module factories call module.NewQueryHandler(), module.NewCommandHandler() etc. directly. Swapping implementations requires modifying the plugin source code.

Fix Direction

Factories should accept constructor functions injected at plugin registration time, or use a secondary registry, so the concrete type can be substituted without touching the plugin. This makes the plugin truly open for extension.

Context

See docs/DEFERRED_ISSUES.md — Architecture section.