Add drawing tools, Mermaid support, and MCP server bridge

- Convert flat project to Cargo workspace (crates/agcanvas, crates/agcanvas-mcp)
- Add drawing layer: rectangles, ellipses, lines, arrows, text with select/move/resize
- Add Mermaid diagram rendering via mermaid-rs-renderer
- Add agcanvas-mcp: MCP server bridge for Claude Code, OpenCode, and Codex
- Add toolbar UI with keyboard shortcuts (V/R/E/L/A/T) and shape interaction
- Add example MCP configs for Claude Code, OpenCode, and Codex
- Update README with full feature docs, MCP setup, and updated architecture

crates/agcanvas-mcp/src/bridge.rs

@@ -0,0 +1,29 @@
+use anyhow::Result;
+use futures_util::{SinkExt, StreamExt};
+use tokio_tungstenite::tungstenite::Message;
+
+pub async fn send_request(ws_url: &str, request_json: &str) -> Result<String> {
+    let (ws_stream, _) = tokio_tungstenite::connect_async(ws_url)
+        .await
+        .map_err(|e| {
+            anyhow::anyhow!(
+                "Cannot connect to agcanvas at {}. Is agcanvas running? Error: {}",
+                ws_url,
+                e
+            )
+        })?;
+
+    let (mut write, mut read) = ws_stream.split();
+
+    // Skip the initial Connected event
+    let _connected = read.next().await;
+
+    write.send(Message::Text(request_json.to_string())).await?;
+
+    match read.next().await {
+        Some(Ok(Message::Text(response))) => Ok(response),
+        Some(Ok(other)) => Err(anyhow::anyhow!("Unexpected message type: {:?}", other)),
+        Some(Err(e)) => Err(anyhow::anyhow!("WebSocket error: {}", e)),
+        None => Err(anyhow::anyhow!("Connection closed before response")),
+    }
+}