[plan] Add Unwrap() methods to custom error types

[github-actions]

Objective

Add Cause error field and Unwrap() error method to 4 custom error types that currently can't participate in Go's error chain inspection.

Context

From Sergo analysis discussion #16458 (hybrid-symbol-error-analysis). Currently, only OperationError correctly implements Unwrap(). The other 4 custom error types break idiomatic Go error handling with errors.Is() and errors.As().

Types to Fix

1. WorkflowValidationError — pkg/workflow/error_helpers.go:14
2. ConfigurationError — pkg/workflow/error_helpers.go:116
3. GitHubToolsetValidationError — pkg/workflow/github_toolset_validation_error.go:16
4. SharedWorkflowError — pkg/workflow/shared_workflow_error.go:16

Reference: Correct Implementation

// OperationError (GOOD - has Unwrap)
type OperationError struct {
    Operation  string
    EntityType string
    EntityID   string
    Cause      error  // ✅ Has Cause field
    Suggestion string
    Timestamp  time.Time
}

func (e *OperationError) Unwrap() error {
    return e.Cause  // ✅ Implements Unwrap
}

Approach

1. Add optional Cause error field to each of the 4 structs
2. Implement Unwrap() error on each struct returning the Cause field
3. Update constructor functions to accept an optional cause parameter (use variadic or a separate WithCause pattern to maintain backward compatibility)
4. Add test cases using errors.Is() and errors.As() to verify error chain inspection works

Files to Modify

• pkg/workflow/error_helpers.go — WorkflowValidationError, ConfigurationError
• pkg/workflow/github_toolset_validation_error.go — GitHubToolsetValidationError
• pkg/workflow/shared_workflow_error.go — SharedWorkflowError

Acceptance Criteria

• All 4 error types have a Cause error field
• All 4 error types implement Unwrap() error
• Constructor functions maintain backward compatibility (existing call sites not broken)
• Tests verify errors.Is() and errors.As() work through error chains
• make agent-finish passes (build, test, lint, fmt)

Generated by Plan Command for issue #discussion #16458

• expires on Feb 20, 2026, 5:34 PM UTC