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>

.changeset/kimi-runtime-support.md

@@ -2,4 +2,4 @@
 type: Added
 pr: 743
 ---
-**Kimi CLI runtime support is now documented and installable** — users can install global GSD Agent Skills with `--kimi --global`, invoke them as `/skill:gsd-*`, and launch the generated custom agent explicitly with `kimi --agent-file`.
+**Kimi CLI runtime support is now documented and installable** — users can install global GSD Agent Skills with `--kimi --global`, invoke them as `/skill:gsd-*`, and launch the generated custom agent explicitly with `kimi --agent-file`. The custom-agent (`--agent-file`) surface targets the legacy/Python `kimi-cli` contract; newer Kimi Code (`@moonshot-ai/kimi-code`) consumes the same `/skill:gsd-*` skills via `--skills-dir` instead.