fix(types): consolidate enhancement alias and fix update command

- Add "enhancement" to util.issueTypeAliases for consistency
- Make types.IssueType.Normalize() case-insensitive and include all aliases
- Fix update.go to normalize type before validation
- Remove duplicate type validation block in update.go

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

internal/util/strings.go

@@ -4,9 +4,10 @@ import "strings"
 
 // issueTypeAliases maps shorthand type names to canonical types
 var issueTypeAliases = map[string]string{
-	"mr":   "merge-request",
-	"feat": "feature",
-	"mol":  "molecule",
+	"mr":          "merge-request",
+	"feat":        "feature",
+	"mol":         "molecule",
+	"enhancement": "feature",
 }
 
 // NormalizeIssueType expands type aliases to their canonical forms.