fix: Remove hardcoded temperature parameter to support models that don't support it (#133)

* Fix: remove hardcoded temperature parameter to support reasoning models

* feat: make temperature configurable via AI_TEMPERATURE env var

- Instead of removing temperature entirely, make it optional via env var
- Set AI_TEMPERATURE=0 for deterministic output (recommended for diagrams)
- Leave unset for models that don't support temperature (e.g., GPT-5.1 reasoning)

* docs: add AI_TEMPERATURE env var documentation

- Update env.example with AI_TEMPERATURE option
- Update README.md configuration section
- Add Temperature Setting section in ai-providers.md

* docs: add TEMPERATURE env var documentation

- Update env.example with TEMPERATURE option
- Update README.md, README_CN.md, README_JA.md configuration sections
- Add Temperature Setting section in ai-providers.md
- Update route.ts to use TEMPERATURE env var

---------

Co-authored-by: dayuan.jiang <jiangdy@amazon.co.jp>

docs/ai-providers.md

@@ -133,6 +133,20 @@ This task requires exceptionally strong model capabilities, as it involves gener
 
 **Note on Ollama**: While Ollama is supported as a provider, it's generally not practical for this use case unless you're running high-capability models like DeepSeek R1 or Qwen3-235B locally.
 
+## Temperature Setting
+
+You can optionally configure the temperature via environment variable:
+
+```bash
+TEMPERATURE=0  # More deterministic output (recommended for diagrams)
+```
+
+**Important**: Leave `TEMPERATURE` unset for models that don't support temperature settings, such as:
+- GPT-5.1 and other reasoning models
+- Some specialized models
+
+When unset, the model uses its default behavior.
+
 ## Recommendations
 
 -   **Best experience**: Use models with vision support (GPT-4o, Claude, Gemini) for image-to-diagram features