Serve MCP at /mcp, keep /health at root

- Route MCP streamable handler to /mcp via http.ServeMux
- Move /health to a dedicated mux route (no auth required)
- Simplify authMiddleware by removing /health special case
- Apply CORS middleware at the mux level
- Ignore nix build result symlink

.gitignore

@@ -1,6 +1,9 @@
 # Binary
 st-pierre
 
+# Nix build result
+result
+
 # Environment
 .env