Improve CLI Help Text and Examples

Enhance the CLI help output with more examples and better formatting.

Tasks:

• Add usage examples to each command's Long description
• Add "EXAMPLES" section showing common use cases
• Improve flag descriptions to be more user-friendly
• Add colorized output to help text using lipgloss
• Add tips section for first-time users

Example:

EXAMPLES:
  # Generate standup for today
  $ devstandup

  # Generate for last week with specific author
  $ devstandup --since "7 days ago" --author "John Doe"

  # Export to markdown file
  $ devstandup --output standup.md

Acceptance Criteria:

• Help text includes at least 5 practical examples
• Flags have clear, beginner-friendly descriptions
• Help output is well-formatted and easy to scan