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/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
 }