Summary

Refactors the monolithic WorkflowExecutor._execute_node_logic node dispatch into modular per-node handlers under backend/app/services/node_execution/nodes/.

What Changed

• Added NodeExecutionContext and a node handler registry for workflow node execution.
• Moved existing node-specific business logic into one handler module per node type without intentional behavior changes.
• Kept WorkflowExecutor responsible for orchestration, retries, tracing, cancellation, pinned data handling, and final NodeResult packaging.
• Added an AGENTS.md directive so future node execution logic goes through modular handlers instead of new executor branches.
• Added a check.sh guard that fails fast when tracked frontend/backend text files exceed 10,000 lines.
• Kept CI E2E on the default Playwright worker mode after the 8-worker/full-parallel run exposed shared-state E2E flakiness.

Impact

This reduces the workflow_executor.py god-file surface and gives each node type a clear backend home while preserving current runtime behavior and existing patch seams used by tests.

Validation

• SECRET_KEY=test-secret-key-for-tests-only-32-bytes uv run pytest tests/test_workflow_executor_branching.py tests/test_json_output_mapper_node.py tests/test_mcp_call_node.py tests/test_websocket_node.py tests/test_discord_node.py tests/test_send_email_node.py tests/test_bigquery_node.py tests/test_github_node.py tests/test_drive_node.py tests/test_linear_node.py tests/test_amazon_s3_node.py
• SECRET_KEY=test-secret-key-for-tests-only-32-bytes ./check.sh
• Full backend suite from ./check.sh: 1696 passed
• ./run_e2e.sh: 66 passed