feat: Extract BID converter from Copilot and prepare plugin migration

- Extract Copilot's BID converter code to src/bid-converter-copilot/ for future use
- Create comprehensive plugin migration request document for Copilot
- Target 13 built-in boxes for plugin conversion (HTTP, GUI, Audio, etc.)
- Preserve existing nyash.toml-based plugin system
- Reorganize docs/説明書/reference/ structure for better organization

local_tests/test_llvm_mock.nyash

@@ -0,0 +1,11 @@
+// Test LLVM mock interpreter
+static box Main {
+    init { result }
+    
+    main() {
+        local a = 10
+        local b = 32
+        local sum = a + b
+        return sum  // Should return 42
+    }
+}