fix: granular reasoning_effort handling for Claude models (#791)

- github.js: split thinking vs reasoning_effort stripping
  - thinking (Claude-native format) still stripped for all Claude on Copilot
  - reasoning_effort now passed through for Opus 4.6 and Sonnet 4.6
  - still stripped for Haiku 4.5 and Opus 4.7 (rejected upstream)
  - reasoning_effort "none" stripped for all models (not all support it)
- openai-to-claude.js: map reasoning_effort → thinking.budget_tokens
  for direct Anthropic backend (none→skip, low→4096, medium→8192,
  high→16384, xhigh→32768)

Previously reasoning_effort was stripped for ALL Claude models,
meaning Opus 4.6 via Copilot never received thinking configuration.

AI-generated commit by Claude Opus 4.6 (Anthropic)

open-sse/translator/request/openai-to-claude.js

@@ -179,6 +179,25 @@ Respond ONLY with the JSON object, no other text.`);
     };
   }
 
+  // Map OpenAI reasoning_effort → Claude thinking.budget_tokens
+  // When client sends reasoning_effort (OpenAI format) but no explicit thinking block,
+  // translate to Claude's native format.
+  if (body.reasoning_effort && !result.thinking) {
+    const effortToBudget = {
+      none:   0,
+      low:    4096,
+      medium: 8192,
+      high:   16384,
+      xhigh:  32768,
+    };
+    const budget = effortToBudget[body.reasoning_effort.toLowerCase()];
+    if (budget === 0) {
+      // none → no thinking
+    } else if (budget) {
+      result.thinking = { type: "enabled", budget_tokens: budget };
+    }
+  }
+
   // Attach toolNameMap to result for response translation
   if (toolNameMap.size > 0) {
     result._toolNameMap = toolNameMap;