feat: configurable report language (default zh-CN)

Add providers.yaml `language` / Action input `language`; prompts and
published summary/inline honor it. Soften fail-closed messaging when
models 429; prefer cohere/nvidia in the first-3 parallel set for dogfood.

docs/dogfood.md

@@ -9,8 +9,9 @@ The Action repo itself runs Gatekeep on its own pull requests.
 | Secret | `OPENROUTER_API_KEY` → step `env` |
 | Trigger | `pull_request` (non-draft) + `workflow_dispatch` |
 | Proven | [PR #2](https://git.potatolab.uk/layue13/gatekeep/pulls/2) got `<!-- gatekeep-review -->`; run 15833 **success** |
+| Report language | `defaults/providers.yaml` → `language: zh-CN` (override with Action `language:`) |
 
-Gitea notes: no `actions/setup-node` mirror — Action requires runner Node ≥18. PyYAML installs via workspace venv (PEP 668). Free OpenRouter models may 429; review still posts (fail-closed tier).
+Gitea notes: no `actions/setup-node` mirror — Action requires runner Node ≥18. PyYAML installs via workspace venv (PEP 668). Free OpenRouter models may 429; summary softens that messaging and prefers cohere/nvidia in the first-3 set.
 
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