Add pipx support with friendly error messages

[cdnsteve]

Summary

• Add _require_sugar_project() helper for consistent error handling across all CLI commands
• Fix sys import bug in add command that was causing UnboundLocalError
• Update README with pipx as the recommended installation method
• Add upgrade/uninstall instructions for pipx users

Changes

• 25+ commands updated to show friendly error when run outside a Sugar project:

  ❌ Not a Sugar project
  
     Could not find: .sugar/config.yaml
  
     Run 'sugar init' to initialize Sugar in this directory.
  

• Core commands: add, list, view, status, run
• Task management: hold, release, remove, update, priority
• Maintenance: logs, stop, cleanup, dedupe
• Advanced: orchestrate, context, thinking, learnings
• Task types: all subcommands
• Debug commands

Test plan

• Verified commands show friendly error in non-Sugar directory (/tmp)
• Verified commands still work correctly in valid Sugar project
• Fixed sys bug that was causing traceback on sugar add

[github-actions]

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[cdnsteve]

I have read the CLA Document and I hereby sign the CLA