feat(store): persist task.review to SQLite

[robotlearning123]

Context

PR #37 added cross-agent diff review (task.review field on Task). Currently task.review is only stored in-memory — it's not persisted to SQLite in store.ts.

What needs to happen

1. Add review column to the tasks table (JSON text, nullable)
2. Update save() to serialize task.review as JSON
3. Update load() to deserialize it back
4. Schema migration for existing databases

Why

Without persistence, review results are lost on restart. API consumers reading historical tasks won't see review data.

Ref: #37

[robotlearning123]

Completed in PR #40 (feat/squash-and-persist). Review field now persists to SQLite via review TEXT column.