[codex] Parse blockedBy strings in s12 task creation

[chaochaoweb3]

Summary

• normalize blockedBy before creating s12 tasks
• accept JSON array strings and Python list-repr strings from tool calls
• reject non-list or non-string dependency inputs before they reach task storage

Fixes #348.

Validation

• python3 -m unittest tests.test_s12_create_task
• python3 -m py_compile s12_task_system/code.py tests/test_s12_create_task.py
• git diff --check

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