docs: add feature 120 plugin system design document

- Plugin SDK (BudgetExperiment.Plugin.Abstractions) for third-party extensions
- Plugin hosting infrastructure for assembly scanning and lifecycle management
- Domain event system wiring existing Transaction._domainEvents scaffolding
- Extension points: API endpoints, import parsers, report builders, Blazor UI
- 9 user stories, 8 implementation phases, phased from MVP to NuGet distribution

Refs: #120

Author: Fortinbra