docs: improve README Getting Started section

[TabishB]

Summary

• Restructured Getting Started section for better readability and flow
• Improved Install & Initialize formatting with clear prerequisites and steps
• Created unified walkthrough example that works for all AI tools (slash commands and AGENTS.md)
• Fixed command references (corrected view vs show usage)
• Removed em dashes throughout for more natural writing style

Changes Made

1. Install & Initialize section:

   • Added prerequisites box for Node.js requirement
   • Split into clear numbered steps with verification commands
   • Added "What happens during initialization" and "After setup" sections

2. Create Your First Change walkthrough:

   • Broke monolithic example into 5 clear workflow stages
   • Added support for both slash command and natural language tools
   • Fixed logical flow (verification happens after creation, not after implementation)
   • Removed unnecessary validation before archiving

3. Command Reference:

   • Fixed incorrect openspec view <change> to correct openspec show <change>
   • Clarified that openspec view is an interactive dashboard

4. General improvements:

   • Replaced all em dashes with more natural alternatives
   • Standardized on npm for package management examples
   • Made examples more concrete and actionable

Test Plan

• Verify README renders correctly on GitHub
• Test that new user onboarding flow makes sense
• Confirm commands in examples are accurate