docs: split CLAUDE.md into focused reference files

Move Common Commands to docs/COMMANDS.md and add docs/CONVENTIONS.md
for frontend patterns (SWR, filters, Mantine, routing, API URLs).
CLAUDE.md now only contains runtime rules and pointers.

docs/COMMANDS.md

@@ -0,0 +1,25 @@
+# Commands
+
+```bash
+bun run dev          # dev server with hot reload (bun --watch src/serve.ts)
+bun run build        # Vite production build
+bun run start        # production server (NODE_ENV=production)
+bun run typecheck    # tsc --noEmit
+bun run lint         # biome check src/
+bun run lint:fix     # biome check --write src/
+
+# Database
+bun run db:migrate   # prisma migrate dev
+bun run db:seed      # seed demo data
+bun run db:generate  # regenerate prisma client
+bun run db:studio    # Prisma Studio GUI
+bun run db:push      # push schema without migration
+
+# Tests
+bun run test              # all tests
+bun run test:unit         # tests/unit/
+bun run test:integration  # tests/integration/ — no server needed
+bun run test:e2e          # tests/e2e/ — requires Lightpanda Docker
+```
+
+Run a single test file: `bun test tests/integration/auth.test.ts`