docs: add clear next-direction roadmap and improvement priorities (EN/ZH)

[Copilot]

The issue asks for a concrete view of the project’s next direction and how to improve it. This PR adds a concise, actionable roadmap to the existing documentation so contributors can align on priorities quickly.

• Roadmap clarity (English README)

  • Added 🧭 Next Direction to define near-term product/learning strategy:
    • adaptive progression
    • richer query diagnostics
    • advanced SQL content expansion
    • quality checks for level/tutorial consistency
    • offline-first reliability

• Execution focus (English README)

  • Added 🔧 Improvement Priorities to rank contribution impact:
    1. learning experience
    2. AI assistant quality/safety/fallback behavior
    3. engineering quality (CI coverage, packaging stability, regression reduction)

• Bilingual parity (Chinese README)

  • Added equivalent sections in doc/README_zh.md:
    • 🧭 下一步方向
    • 🔧 改进优先级
  • Kept content aligned with English roadmap to avoid strategy drift.

## 🧭 Next Direction
- **Adaptive learning path** — Recommend the next level based on pass/fail history and common mistakes
- **Stronger answer diagnostics** — Explain *why* a query is wrong (syntax, logic, edge cases), not only pass/fail

## 🔧 Improvement Priorities
1. **Learning experience**
2. **AI assistant quality**
3. **Engineering quality**

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• registry.npmmirror.com
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node node /home/REDACTED/work/_temp/ghcca-node/node/bin/npm install (dns block)
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node node /usr/local/bin/yarn install --frozen-lockfile (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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