feat(formula): add untracked status for formulas without .installed.json

When upgrading gt on an existing installation without .installed.json,
formulas that exist but don't match embedded were incorrectly marked as
"modified" (implying user customization). Now they're marked "untracked"
and are safe to update since there's no record of user modification.

This improves the upgrade experience:
- "modified" = tracked file user changed (skip update)
- "untracked" = file exists but not tracked (safe to update)

Adds 3 new tests for untracked scenarios.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/doctor/formula_check.go

@@ -38,7 +38,7 @@ func (c *FormulaCheck) Run(ctx *CheckContext) *CheckResult {
 	}
 
 	// All good
-	if report.Outdated == 0 && report.Missing == 0 && report.Modified == 0 && report.New == 0 {
+	if report.Outdated == 0 && report.Missing == 0 && report.Modified == 0 && report.New == 0 && report.Untracked == 0 {
 		return &CheckResult{
 			Name:    c.Name(),
 			Status:  StatusOK,
@@ -63,6 +63,9 @@ func (c *FormulaCheck) Run(ctx *CheckContext) *CheckResult {
 		case "new":
 			details = append(details, fmt.Sprintf("  %s: new formula available", f.Name))
 			needsFix = true
+		case "untracked":
+			details = append(details, fmt.Sprintf("  %s: untracked (will update)", f.Name))
+			needsFix = true
 		}
 	}
 
@@ -83,6 +86,9 @@ func (c *FormulaCheck) Run(ctx *CheckContext) *CheckResult {
 	if report.New > 0 {
 		parts = append(parts, fmt.Sprintf("%d new", report.New))
 	}
+	if report.Untracked > 0 {
+		parts = append(parts, fmt.Sprintf("%d untracked", report.Untracked))
+	}
 	if report.Modified > 0 {
 		parts = append(parts, fmt.Sprintf("%d modified", report.Modified))
 	}