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

examples/mcp-configs/opencode.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://opencode.ai/config.schema.json",
+  "mcpServers": {
+    "agcanvas": {
+      "command": "agcanvas-mcp",
+      "args": ["--port", "9876"]
+    }
+  }
+}