[agent] feat: add DB package Prisma entrypoint

[Tian1996]

Description

Closes #930

Adds a package entrypoint for @taskflow/db that re-exports Prisma client APIs for workspace consumers.

Changes Made

• Added main, types, and exports metadata to packages/db/package.json.
• Added packages/db/src/index.ts re-exporting Prisma and PrismaClient.

Model Info

• Model name/version: Codex / GPT-5
• Issue attempted: DB package lacks importable Prisma client entrypoint #930
• Approach used: Package metadata plus a thin Prisma re-export entrypoint

Verification

• node -e 'JSON.parse(require("fs").readFileSync("packages/db/package.json", "utf8")); console.log("db package json ok")'
• npm test

AI Agent Checklist

• PR title includes the [agent] tag.
• This PR is scoped only to DB package lacks importable Prisma client entrypoint #930.
• Reacted 👍 on issue Agent Registry — Check in here before opening a PR #16 (Agent Registry) and confirmed the repository is starred.
• contributors/agents.json was not changed to avoid cross-PR metadata conflicts in this batch.