refactor: move brother_node development artifact to dev/test-nodes subdirectory

Development Artifact Cleanup:
✅ BROTHER_NODE REORGANIZATION: Moved development test node to appropriate location
- dev/test-nodes/brother_node/: Moved from root directory for better organization
- Contains development configuration, test logs, and test chain data
- No impact on production systems - purely development/testing artifact

✅ DEVELOPMENT ARTIFACTS IDENTIFIED:
- Chain ID: aitbc-brother-chain (test/development chain)
- Ports: 8010 (P2P) and 8011 (RPC) - different from production
- Environment: .env file with test configuration
- Logs: rpc.log and node.log from development testing session (March 15, 2026)

✅ ROOT DIRECTORY CLEANUP: Removed development clutter from production directory
- brother_node/ moved to dev/test-nodes/brother_node/
- Root directory now contains only production-ready components
- Development artifacts properly organized in dev/ subdirectory

DIRECTORY STRUCTURE IMPROVEMENT:
📁 dev/test-nodes/: Development and testing node configurations
🏗️ Root Directory: Clean production structure with only essential components
🧪 Development Isolation: Test environments separated from production

BENEFITS:
✅ Clean Production Directory: No development artifacts in root
✅ Better Organization: Development nodes grouped in dev/ subdirectory
✅ Clear Separation: Production vs development environments clearly distinguished
✅ Maintainability: Easier to identify and manage development components

RESULT: Successfully moved brother_node development artifact to dev/test-nodes/ subdirectory, cleaning up the root directory while preserving development testing environment for future use.

dev/env/node_modules/@openzeppelin/contracts/interfaces/draft-IERC7821.sol

@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v5.4.0) (interfaces/draft-IERC7821.sol)
+
+pragma solidity >=0.5.0;
+
+/**
+ * @dev Interface for minimal batch executor.
+ */
+interface IERC7821 {
+    /**
+     * @dev Executes the calls in `executionData`.
+     * Reverts and bubbles up error if any call fails.
+     *
+     * `executionData` encoding:
+     * - If `opData` is empty, `executionData` is simply `abi.encode(calls)`.
+     * - Else, `executionData` is `abi.encode(calls, opData)`.
+     *   See: https://eips.ethereum.org/EIPS/eip-7579
+     *
+     * Supported modes:
+     * - `bytes32(0x01000000000000000000...)`: does not support optional `opData`.
+     * - `bytes32(0x01000000000078210001...)`: supports optional `opData`.
+     *
+     * Authorization checks:
+     * - If `opData` is empty, the implementation SHOULD require that
+     *   `msg.sender == address(this)`.
+     * - If `opData` is not empty, the implementation SHOULD use the signature
+     *   encoded in `opData` to determine if the caller can perform the execution.
+     *
+     * `opData` may be used to store additional data for authentication,
+     * paymaster data, gas limits, etc.
+     *
+     * For calldata compression efficiency, if a Call.to is `address(0)`,
+     * it will be replaced with `address(this)`.
+     */
+    function execute(bytes32 mode, bytes calldata executionData) external payable;
+
+    /**
+     * @dev This function is provided for frontends to detect support.
+     * Only returns true for:
+     * - `bytes32(0x01000000000000000000...)`: does not support optional `opData`.
+     * - `bytes32(0x01000000000078210001...)`: supports optional `opData`.
+     */
+    function supportsExecutionMode(bytes32 mode) external view returns (bool);
+}