docs: expand 从零到 AI Agent 专家 course

[franksong2702]

What changed

• Expand all 12 chapters of textbook-zero-to-agent/ from outline-style notes into full teaching text.
• Add principles, practical analogies, runnable examples, failure cases, and beginner-oriented explanations.
• Update the course index with the expansion version note.

Scope

• 13 course Markdown files changed.
• Lab packages and other knowledge-base areas are unchanged.

Validation

• python3 _tools/kb_health_check.py
• python3 -m py_compile _tools/*.py
• Course structure check: 12 chapters indexed; code fences balanced.
• git diff --check

All checks passed locally.