Add non-empty validation to workflow intent field

[Copilot]

The huh.NewText() intent field in interactive.go accepted empty strings while the workflow name field already enforced non-empty input — an inconsistency flagged in the Terminal Stylist audit.

Changes

• pkg/cli/validators.go — Added ValidateWorkflowIntent function that rejects empty/whitespace-only input and enforces a 20-character minimum for meaningful instructions
• pkg/cli/interactive.go — Wired .Validate(ValidateWorkflowIntent) onto the intent huh.NewText() field
• pkg/cli/validators_test.go — Added TestValidateWorkflowIntent and TestValidateWorkflowIntent_EdgeCases covering empty, whitespace-only, too-short, and valid inputs

huh.NewText().
    Title("Describe what this workflow should do:").
    Value(&b.Intent).
    Validate(ValidateWorkflowIntent),  // rejects empty + < 20 chars

func ValidateWorkflowIntent(s string) error {
    trimmed := strings.TrimSpace(s)
    if len(trimmed) == 0 {
        return errors.New("workflow instructions cannot be empty")
    }
    if len(trimmed) < 20 {
        return errors.New("please provide at least 20 characters of instructions")
    }
    return nil
}

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