🔧 Simplify workflow add command and remove package management

[dsyme]

Summary

• Simplified gh aw add command to use a new, more intuitive workflow specification format
• Removed legacy package management commands (install, uninstall, list --packages)
• Updated CLI workflow listing to provide clearer guidance

Key Changes

• New workflow specification format: owner/repo/workflow-name[@version]
• Implicit workflows/ directory for workflow lookup
• Simplified workflow addition process
• Removed separate install/uninstall commands
• Updated documentation and examples
• Improved error handling for workflow specifications

Breaking Changes

• Removed -r flag for workflow addition
• Removed install, uninstall, and package listing commands
• New workflow specification requires full repository context

Migration Guide

Update existing workflow add commands from:

gh aw add weekly-research -r githubnext/agentics

to:

gh aw add githubnext/agentics/weekly-research