Fix: Centralize BasePrompt and resolve import issues across notebooks

[ManojRamani]

This commit addresses issue #21 by fixing the BasePrompt import issues and improving the notebook infrastructure.

Changes:

• Centralized BasePrompt: All notebooks now import from agentic_platform.core.models.prompt_models instead of local utils
• Added missing methods to central BasePrompt:
  • to_bedrock_messages() for message formatting
  • to_bedrock_system() for system prompt formatting
• Removed deprecated labs/module2/notebooks/utils/base_prompt.py
• Fixed import order in agentcore_tool_browser notebook
• Added nova-act>=0.1.0 dependency to pyproject.toml
• Updated module4 README with API key setup instructions for Tavily and Nova Act
• Added .gitattributes for nbstripout to handle notebook metadata
• Fixed ActResult handling in browser tool for proper result extraction

Notebooks updated:

• labs/module2/notebooks/3_routing.ipynb
• labs/module2/notebooks/4_parallelization.ipynb
• labs/module2/notebooks/5_orchestrator.ipynb
• labs/module2/notebooks/6_evaluator_optimizer.ipynb
• labs/module4/notebooks/2_multi-agent-delegation.ipynb
• labs/module4/notebooks/5_agentcore_tool_browser.ipynb

Issue #, if available:

Description of changes:

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