feat: add @seekdb/prisma-adapter and ORM examples (seekdb-drizzle, seekdb-prisma)

[dengfuping]

Summary

• New: @seekdb/prisma-adapter (Prisma driver adapter for seekdb Embedded) and two ORM examples (seekdb-drizzle, seekdb-prisma).
• Unify: Env vars to SEEKDB_*; docs put embedded mode first, add OceanBase/tenant section, expand Packages table; remove TEST_CONFIG_OB; factory only passes tenant when set.
• Pre dependent PR: feat(seekdb): add client.execute API for raw SQL execution #20 should be merged first.

Changes

1. @seekdb/prisma-adapter

• Package: packages/prisma-adapter — adapter (client.execute), conversion, tests, README.
• Usage: new PrismaSeekdb(client) → new PrismaClient({ adapter }); Prisma ≥6, seekdb Embedded only.

2. ORM examples

• seekdb-drizzle: Server = mysql2 same DB; Embedded = mysql-proxy + client.execute().
• seekdb-prisma: Server = DATABASE_URL; Embedded = @seekdb/prisma-adapter.
• Each: index.ts / index-embedded.ts, shared.ts, README, package.json.

3. Docs & config

• README / seekdb README: Packages table (Core / Integrations / Embeddings, ordered); Examples (basic + ORM); Client Connection & Database Management — embedded first, then Server, then OceanBase mode (tenant); Quick Start order.

4. Env & tenant

• Env: All connection env unified to SEEKDB_HOST, SEEKDB_PORT, SEEKDB_USER, SEEKDB_PASSWORD, SEEKDB_DATABASE (CI, test-utils, .env.example). No SEEKDB_TENANT; tenant fixed as "sys" in code where needed.
• Factory: Pass tenant to client only when explicitly set (OceanBase); seekdb server uses internal default.

Verify

pnpm install && pnpm build && pnpm test
pnpm --filter seekdb-drizzle-example run start:embedded
pnpm --filter seekdb-prisma-example run db:generate && pnpm --filter seekdb-prisma-example run start:embedded