Consolidate execution status and timing into execution_data.json

[stefanpie]

Execution results are currently split across timing data, success artifacts, and marker files such as error__{flow_name}.txt and timeout__{flow_name}.txt.

Create a unified execution_data.json format containing:

• Execution status: success, error, or timeout
• Execution-time data
• Flow name
• Relevant output or error details

Replace current artifacts (error__{flow_name}.txt, timeout__{flow_name}.txt, and execution_time_data.json) with execution_data.jsonas the authoritative execution record.