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>

app/api/chat/route.ts

@@ -418,7 +418,9 @@ IMPORTANT: Keep edits concise:
                 }),
             },
         },
-        temperature: 0,
+        ...(process.env.TEMPERATURE !== undefined && {
+            temperature: parseFloat(process.env.TEMPERATURE),
+        }),
     })
 
     return result.toUIMessageStreamResponse()