feat(update): add --type flag to bd update command (GH#522)

Allow changing an issue type (bug, feature, task, epic, chore) via the
bd update command. The storage layer already supported issue_type in
allowedUpdateFields, this just exposes it through the CLI.

Changes:
- Add --type/-t flag to updateCmd in show.go
- Add IssueType field to UpdateArgs in protocol.go
- Handle issue_type in updatesFromArgs in server_issues_epics.go
- Add validation using ParseIssueType before update

Example usage:
  bd update ab-xyz --type epic

Fixes: #522

internal/rpc/protocol.go

@@ -87,6 +87,7 @@ type UpdateArgs struct {
 	Assignee           *string  `json:"assignee,omitempty"`
 	ExternalRef        *string  `json:"external_ref,omitempty"` // Link to external issue trackers
 	EstimatedMinutes   *int     `json:"estimated_minutes,omitempty"` // Time estimate in minutes
+	IssueType          *string  `json:"issue_type,omitempty"`        // Issue type (bug, feature, task, epic, chore)
 	AddLabels          []string `json:"add_labels,omitempty"`
 	RemoveLabels       []string `json:"remove_labels,omitempty"`
 	SetLabels          []string `json:"set_labels,omitempty"`

internal/rpc/server_issues_epics.go

@@ -73,6 +73,9 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.EstimatedMinutes != nil {
 		u["estimated_minutes"] = *a.EstimatedMinutes
 	}
+	if a.IssueType != nil {
+		u["issue_type"] = *a.IssueType
+	}
 	return u
 }