fix(product-docs): limit sync PR body length

[Terry-Mao]

Summary

• Cap generated product docs sync PR body/comment markdown before calling GitHub.
• Summarize only recent ledger decisions and truncate oversized reason/patch fields.
• Document the length guard in product docs raw/wiki.

Validation

• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/aicodingflow-tests -p 'test_product_docs_sync.py'
• PYTHONDONTWRITEBYTECODE=1 python3 -m unittest discover -s .github/aicodingflow-tests
• PYTHONPYCACHEPREFIX=/tmp/aicodingflow-pycache python3 -m py_compile .github/scripts/write_product_docs_sync_pr_body.py .github/aicodingflow-tests/test_product_docs_sync.py
• git diff --check

Fixes #257

[github-actions]

未发现需要阻塞合并的问题。