Add plugin development guide with boilerplate template

[elijahr]

Creating a new plugin requires understanding StateMachinePlugin, Timeline, interaction recording, and assertion helpers. Write a step-by-step guide (docs/guides/writing-plugins.md already exists but could be expanded) with:

• A copy-paste plugin template
• Checklist of required methods (`record()`, `assertable_fields()`, proxy class)
• Common pitfalls (auto-assert prohibition, assertable_fields coverage)
• Reference implementations (LoggingPlugin as simplest, RedisPlugin as stateful)