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

Allows changing issue type (task/epic/bug/feature/chore) via bd update --type.
Storage layer already supported it, this adds CLI and RPC support.

Fixes GH#522.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

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"`