docs(plan-2): document relative-imports-in-source rule for feature packages

Discovered during execution: when downstream packages like @repo/core-api
typecheck and follow imports into a feature, they don't have the feature's
@/ alias and fail with TS2307. Source files use relative imports;
@/ is reserved for test files within the feature's own context.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/superpowers/plans/2026-05-04-plan-2-blog-feature.md

@@ -138,7 +138,7 @@ The domain `Article.content` field is also widened: the existing entity has `con
 }
 ```
 
-> Note: `@/*` path alias is feature-local (resolves to `packages/blog/src/*`). This is the same pattern `packages/core/tsconfig.json` already uses.
+> Note: `@/*` path alias is defined here for use in **test files** only. Source files inside `src/` should use **relative imports** (`../../entities/article` etc.). Reason: when a downstream package like `@repo/core-api` typechecks and follows imports into `@repo/blog`, it descends into blog's source files but doesn't have blog's `@/` alias in its own tsconfig — causing TS2307 errors. Relative imports in source files avoid this. Tests stay in `tests/` or alongside source where they use the same package context, so `@/` works there. Same convention applies to all feature packages in subsequent plans.
 
 - [ ] **Step 3: Create turbo.json**