refactor: Add plugin API abstraction layer

[nigel-dev]

Source

Audit report — Section 7: Technical Debt > Architecture

Description

MC directly uses the OpenCode plugin SDK APIs (client.session.prompt(), client.session.get(), hook registration, etc.) throughout the codebase without an abstraction layer. This couples MC tightly to the specific OpenCode SDK version and makes it impossible to:

• Run MC logic in tests without the full plugin runtime
• Port MC to other AI coding tool ecosystems
• Mock the plugin API cleanly in unit tests

Current State

• index.ts directly uses client.session.prompt(), client.session.get(), hook registration
• notifications.ts, compaction.ts, commands.ts all depend directly on SDK types
• No interface abstraction for the plugin SDK

Proposed Solution

1. Define a PluginHost interface covering MC's SDK usage: notifications, hooks, session state, commands
2. Implement OpenCodePluginHost wrapping the real SDK
3. MC core logic depends on PluginHost, not the SDK directly
4. Tests can use MockPluginHost
5. Future: ClaudeCodePluginHost, CursorPluginHost, etc.

Priority

P3 — architectural improvement. Not blocking functionality, but improves testability and future portability.

Relationship to Other Issues

• Complements refactor: Add dependency injection for gitCommand, tmux, and Bun.spawn #45 (DI for git/tmux/spawn) — both improve testability through abstraction
• Aids test: Add test coverage for untested core files #25 (test coverage) — makes index.ts and commands.ts testable