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

src/bid-converter-copilot/error.rs

@@ -0,0 +1,80 @@
+/// BID-1 Standard Error Codes
+#[repr(i32)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum BidError {
+    /// Operation successful
+    Success = 0,
+    
+    /// Buffer too small (need to call again with larger buffer)
+    ShortBuffer = -1,
+    
+    /// Invalid type ID
+    InvalidType = -2,
+    
+    /// Invalid method ID
+    InvalidMethod = -3,
+    
+    /// Invalid arguments
+    InvalidArgs = -4,
+    
+    /// Plugin internal error
+    PluginError = -5,
+    
+    /// Memory allocation failed
+    OutOfMemory = -6,
+    
+    /// UTF-8 encoding error
+    InvalidUtf8 = -7,
+    
+    /// Handle not found
+    InvalidHandle = -8,
+    
+    /// Version mismatch
+    VersionMismatch = -9,
+}
+
+impl BidError {
+    /// Convert from raw i32
+    pub fn from_raw(code: i32) -> Self {
+        match code {
+            0 => BidError::Success,
+            -1 => BidError::ShortBuffer,
+            -2 => BidError::InvalidType,
+            -3 => BidError::InvalidMethod,
+            -4 => BidError::InvalidArgs,
+            -5 => BidError::PluginError,
+            -6 => BidError::OutOfMemory,
+            -7 => BidError::InvalidUtf8,
+            -8 => BidError::InvalidHandle,
+            -9 => BidError::VersionMismatch,
+            _ => BidError::PluginError, // Unknown errors map to plugin error
+        }
+    }
+    
+    /// Get human-readable error message
+    pub fn message(&self) -> &'static str {
+        match self {
+            BidError::Success => "Operation successful",
+            BidError::ShortBuffer => "Buffer too small, call again with larger buffer",
+            BidError::InvalidType => "Invalid type ID",
+            BidError::InvalidMethod => "Invalid method ID",
+            BidError::InvalidArgs => "Invalid arguments",
+            BidError::PluginError => "Plugin internal error",
+            BidError::OutOfMemory => "Memory allocation failed",
+            BidError::InvalidUtf8 => "Invalid UTF-8 encoding",
+            BidError::InvalidHandle => "Handle not found",
+            BidError::VersionMismatch => "BID version mismatch",
+        }
+    }
+}
+
+impl std::fmt::Display for BidError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{} (code: {})", self.message(), *self as i32)
+    }
+}
+
+impl std::error::Error for BidError {}
+
+/// Result type for BID operations
+pub type BidResult<T> = Result<T, BidError>;