[codex] Remove stale mcp_log_dir reference

[sergunchik]

Summary

Removes a stale mcp_log_dir reference from TaskMetadata.__post_init__.

Root Cause

mcp_log_dir existed in the initial MCP log implementation, but commit fd4100b migrated logging to per-task CLI logs via CLI_LOG_PATH. That commit removed the mcp_log_dir / mcp_log_name dataclass fields, serialization fields, config examples, and MCP stats plumbing, but left one normalization block behind in __post_init__.

Because the dataclass no longer defines mcp_log_dir, constructing any TaskMetadata currently raises:

AttributeError: 'TaskMetadata' object has no attribute 'mcp_log_dir'

Impact

This fixes task construction for run, batch, from-folder, and LiveProveBench runs. It does not change the current logging interface; per-task logs continue to use CLI_LOG_PATH=<result_dir>/<task_id>/cli.log.

Validation

• Confirmed history: mcp_log_dir existed in 32825eb and was removed by the CLI log migration in fd4100b.
• Confirmed current references before the fix: git grep showed mcp_log_dir only in the stale scripts/task.py block.
• Ran constructor smoke test.
• Ran script parse check.