Add feature to suggest starting prompts for codegen

[codegen-sh]

Description

This PR adds a feature to suggest starting prompts for the codegen CLI agent, making it more user-friendly and helping users discover its capabilities.

Changes

• Added a list of suggested starting prompts as a constant in the agent command module
• Enhanced the welcome message to display the top 3 suggested prompts in a visually appealing panel
• Added a --help-prompts flag to the agent command to show all suggested prompts and exit
• Implemented a "help" command that users can type at any time to see all suggested prompts
• Made the help command respond to various forms like "help", "--help", "man", "?", and "examples"

Benefits

• Makes it easier for new users to get started with codegen
• Surfaces useful commands that users might not discover on their own
• Provides a consistent way to access help and examples
• Improves the overall user experience

Testing

Tested the changes locally to ensure:

• The welcome message displays correctly with suggested prompts
• The help command works at any point in the conversation
• The --help-prompts flag works as expected

Fixes Linear issue: Add feature to suggest starting prompts for codegen

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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