feat: split builtin task type definitions

[joy7758]

Summary

• add demos/task_types.py to hold builtin persona and task-type definitions
• slim demos/task_registry.py down to registry primitives plus builtin loader wiring
• keep stage handler plugins and workflow behavior unchanged while moving task metadata out of the registry module

Validation

• python3 -m compileall demos/task_registry.py demos/task_types.py demos/persona_workflow_demo.py demos/stage_handlers.py
• python3 demos/persona_workflow_demo.py
• python3 demos/persona_workflow_demo.py --task-input demos/product_design_task.json
• python3 demos/persona_workflow_demo.py --task-input demos/ux_review_task.json
• python3 demos/persona_workflow_demo.py --task-input /tmp/pop_legacy_task.json
• schema validation for market_research_task.json, product_design_task.json, and ux_review_task.json

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Break circular import between registry and task types

Importing demos.task_types now fails because task_types.py imports demos.task_registry, and task_registry.py immediately calls load_builtin_registry() which does from demos.task_types import BUILTIN_PERSONAS, BUILTIN_TASK_TYPES before task_types has finished initializing. In this code path (e.g., python3 -c "import demos.task_types"), Python raises ImportError for a partially initialized module, so the new split module cannot be imported directly.

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