fix(decision): replace fullwidth space (U+3000) in JSON

Critical bug: AI can output fullwidth space (　U+3000) between brackets:
Input:  ［　｛"symbol":"BTCUSDT"｝］
        ↑ ↑ fullwidth space

After previous fix:
        [　{"symbol":"BTCUSDT"}]
         ↑ fullwidth space remained!

Result: validateJSONFormat failed because:
- Checks "[{" (no space) ❌
- Checks "[ {" (halfwidth space U+0020) ❌
- AI output "[　{" (fullwidth space U+3000) ❌

Solution: Replace fullwidth space → halfwidth space
- 　(U+3000) → space (U+0020)

This allows existing validation logic to work:
strings.HasPrefix(trimmed, "[ {") now matches ✅

Why fullwidth space?
- Common in CJK text editing
- AI trained on mixed CJK content
- Invisible to naked eye but breaks JSON parsing

Test case:
Input:  ［　｛"symbol":"BTCUSDT"｝］
Output: [ {"symbol":"BTCUSDT"}]
Validation: ✅ PASS

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: tinkle-community <tinklefund@gmail.com>

decision/engine.go

@@ -487,6 +487,9 @@ func fixMissingQuotes(jsonStr string) string {
 	jsonStr = strings.ReplaceAll(jsonStr, "〕", "]") // CJK右龟壳括号 U+3015
 	jsonStr = strings.ReplaceAll(jsonStr, "、", ",") // CJK顿号 U+3001
 
+	// ⚠️ 替换全角空格为半角空格（JSON中不应该有全角空格）
+	jsonStr = strings.ReplaceAll(jsonStr, "　", " ") // U+3000 全角空格
+
 	return jsonStr
 }