docs(agents): add per-package AGENTS.md for all core-* packages

packages/core-shared/AGENTS.md

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+# AGENTS.md — core-shared
+
+Generic, reusable primitives with **zero business knowledge**. This package is the foundation for all other packages and exports utilities, Payload field/hook definitions, and tRPC initialization.
+
+## Responsibilities
+
+- **Generic primitives** — environment helpers, date utilities, type guards
+- **Payload utilities** — field definitions (slug, SEO), blocks (CTA), access controls (is-admin), hooks (slugify, publish timestamp)
+- **tRPC platform** — `initTRPC.create()`, shared context factory, procedure builders
+- **No business domain knowledge** — no awareness of articles, users, media, or any feature
+
+## Must NOT import
+
+- Any feature package (`@repo/auth`, `@repo/blog`, etc.)
+- Any app package
+- Framework-specific code (Next.js, TanStack React Query)
+
+## Public exports
+
+From `package.json`:
+- `.` — all utilities, Payload exports, tRPC init
+- `./payload` — Payload field/hook/block utilities only
+- `./trpc/init` — tRPC initialization only
+- `./trpc/context` — tRPC context factory only
+
+## Test conventions
+
+- Tests colocated: `src/lib/slug-field.ts` → `src/lib/slug-field.test.ts`
+- Vitest environment: `node`
+- Alias: `@/` resolves to `src/`
+- Run: `pnpm test --filter @repo/core-shared`
+
+Covered areas:
+- Slug field generation + validation
+- Payload hooks (publish-at timestamp, slugify-if-missing)
+- Access control helpers