feat: improve error messages with actionable next steps

[brigleb]

Summary

Enhance error messages to include actionable suggestions and next steps, helping users recover from errors more easily.

Current Behavior

Error: no todo list specified. Use --list flag or run 'bc4 todo set' to set a default

Proposed Behavior

Error: No default todo list configured

To fix this, you can either:
  1. Set a default: bc4 todo set
  2. Specify a list: bc4 todo add "Task" --list "My List"
  3. View available lists: bc4 todo lists

Learn more: bc4 todo --help

Implementation

1. Create helper functions in internal/errors/ for common error scenarios
2. Include contextual suggestions based on the operation
3. Add "Learn more" links to relevant help

Common Error Scenarios to Improve

• No project selected
• No todo list selected
• Authentication expired
• Resource not found
• Permission denied
• Invalid URL format

Files to Modify

• Create internal/errors/suggestions.go
• Update error handling in cmd/todo/add.go
• Update error handling in cmd/card/add.go
• Update error handling in cmd/message/post.go
• etc.

Reference

GitHub CLI provides excellent error messages with suggestions:

gh pr create
? No default remote found. Would you like to set origin as the default? Yes

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Extracted from PR #79 discoverability analysis