feat: integrate dynamic task input into persona workflow

[joy7758]

Summary:

• add a task input JSON file for the persona workflow demo
• make the workflow read dynamic task data and shape each persona stage from that input
• write the final structured deliverable to demos/generated/*.json

Validation:

• ran python3 demos/persona_workflow_demo.py
• verified the generated deliverable JSON file was written

Notes:

• this PR is stacked on top of feat: add structured deliverable output to persona workflow #11 because feat: add persona runtime demo with multiple personas #8, feat: add multi-persona workflow for collaborative task #9, and feat: add structured deliverable output to persona workflow #11 are not merged into main yet

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Guard against empty feature lists in campaign hook

When task_input.product.features is present but empty (e.g., "features": []), indexing product.get('features', ['key features'])[0] raises IndexError and aborts the workflow before the marketing result/final deliverable is produced. Since this commit introduces dynamic task input, empty feature arrays are a realistic input shape and should fall back to a default hook instead of crashing.

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