Summary

• restores the Prompt: Chinese reasoning — assistant thinks in English even when user writes in Chinese #588 contract so the latest user message chooses the natural language for both reasoning_content and final replies
• keeps ## Environment lang as an ambiguity fallback instead of letting English system locale override Chinese user turns
• bumps workspace/npm binary version to 0.8.20 and documents the release note

Test plan

• ./scripts/release/check-versions.sh
• cargo test -p deepseek-tui language_mirroring
• cargo test -p deepseek-tui package_version_is_current_hotfix_release
• cargo test -p deepseek-tui environment_block
• cargo fmt --all --check
• git diff --check
• Live V4 smoke with lang: en and Chinese user text: reasoning_content stats were 35 Han chars, 0 Latin chars; final answer was Chinese
• Source dispatcher smoke under LC_ALL=en_US.UTF-8: ./target/debug/deepseek exec --model deepseek-v4-pro --auto --json ... returned Chinese final output

Closes #588