docs(#743): address Kimi review — runtime count + support boundary

Address remaining review feedback on PR #743:

- Bump supported-runtime count 15 -> 16 across user-facing READMEs
  (docs/README.md + ja-JP/zh-CN translations) now that Kimi is added.
- Add an explicit support-boundary note in the Kimi CLI install section:
  the kimi --agent-file custom-agent contract targets legacy/Python
  kimi-cli; newer npm Kimi Code (@moonshot-ai/kimi-code) rejects
  --agent-file and consumes the same /skill:gsd-* skills via --skills-dir.
- Mirror the boundary in the changeset so release notes are unambiguous.

Historical ADR/changeset/INVENTORY references to '15 runtimes' are left
intact as point-in-time records.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/zh-CN/README.md

@@ -15,7 +15,7 @@
 
 ## How-to guides
 
-- [在你的运行时上安装](how-to/install-on-your-runtime.md) — 适用于全部 15 个受支持运行时的安装步骤
+- [在你的运行时上安装](how-to/install-on-your-runtime.md) — 适用于全部 16 个受支持运行时的安装步骤
 - [讨论一个阶段](how-to/discuss-a-phase.md) — 在规划开始前记录实现决策
 - [规划一个阶段](how-to/plan-a-phase.md) — 执行调研、分解工作并验证计划质量
 - [执行一个阶段](how-to/execute-a-phase.md) — 使用全新上下文的子代理以并行波次运行计划