docs(adr): add ADRs 006-009 for vertical refactor

docs/decisions/adr-006-vertical-feature-packages.md

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+# ADR-006: Vertical Feature Packages
+
+**Status:** Accepted
+**Date:** 2026-05-04
+
+## Context
+
+The original template organized packages by architectural layer: `core` (all domains together), `api` (all routers), `ui` (all components). As features grow, shared code accumulates and cross-feature dependencies become implicit.
+
+## Decision
+
+Reorganize by business capability. Each feature owns a vertical slice from entities through UI. `core-*` packages host only non-business concerns (DI, shared types, UI primitives).
+
+**Result:** 5 feature packages (`auth`, `blog`, `media`, `marketing-pages`, `navigation`) + 5 core packages (`core-shared`, `core-cms`, `core-api`, `core-trpc`, `core-ui`).
+
+## Consequences
+
+- Features evolve independently without coordinating with shared code
+- Cross-feature coupling is visible at the package-graph level (ESLint enforces it)
+- Per-feature DI containers eliminate symbol collisions
+- New team members can understand a feature completely by reading one package
+
+## Alternatives Considered
+
+- Horizontal layers (kept): Simpler initially, but scales to implicit hidden dependencies
+- Monolithic single package: No modularity, impossible to reason about at scale
+- Feature shells + shared core: Hybrid approach tried by many teams; creates "dump" in core that nobody owns