[plan] Add empathy and learning links to validation error messages

[github-actions]

Objective

Transform validation error messages from technical rejections into empathetic, educational guidance that helps developers learn and fix issues quickly.

Context

Parent Issue: This is part of the user experience improvements tracked in the parent issue.

Source: Delight Audit identified that validation errors in pkg/workflow/*validation*.go files are technically correct but lack empathy and learning resources, creating friction for first-time users.

Current State: Errors like this are common:

return fmt.Errorf("invalid engine: %s. Valid engines are: copilot, claude, codex, custom. Example: engine: copilot", engineID)

Target State: Errors should be empathetic, educational, and actionable:

return fmt.Errorf("🤔 Hmm, we don't recognize the engine '%s'.\n\nValid options are:\n  • copilot (GitHub Copilot)\n  • claude (Anthropic Claude)\n  • codex (OpenAI Codex)\n  • custom (your own engine)\n\nExample:\n  engine: copilot\n\nNeed help choosing? See: https://githubnext.github.io/gh-aw/reference/engines/", engineID)

Approach

1. Audit all validation error messages in pkg/workflow/*validation*.go files
2. For each error, add:
   • Empathetic opening with emoji and acknowledgment
   • Context explaining why the rule exists (security, best practices)
   • Specific fix examples
   • Learning resource links where applicable
   • Conversational, helpful tone (not blaming)
3. Follow the error message style guide at skills/error-messages.md
4. Test changes by triggering validation errors with invalid workflow configs

Files to Review

• pkg/workflow/engine_validation.go
• pkg/workflow/dangerous_permissions_validation.go
• pkg/workflow/mcp_config_validation.go
• All other pkg/workflow/*validation*.go files

Acceptance Criteria

• All validation error messages reviewed and enhanced
• Errors include empathetic opening (emoji + acknowledgment)
• Errors explain why the rule exists
• Errors provide specific fix examples
• Errors include learning resource links where applicable
• Tone is conversational and helpful
• Manual testing confirms improved error output
• Delight rating target: 😐 Neutral → 🙂 Good

Why This Matters

User Impact: Validation errors are teaching moments - first-time users encounter them during workflow setup. This is a critical opportunity to build confidence or create frustration.

Frequency: High - every developer setting up workflows encounters validation errors.

AirBnB Principles: Conversational, Trust and Safety

AI generated by Plan Command for discussion #10343

[github-actions]

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The Copilot agent will analyze the issue and create a pull request with the fix.

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