Add description parameter to bd update command and MCP server

Fixes #114 and #122 by adding --description/-d flag to bd update CLI
and description parameter to MCP update_issue tool.

Changes:
- CLI: Added --description flag to updateCmd
- RPC: Added Description field to UpdateArgs
- Daemon: Updated updatesFromArgs to handle description
- MCP: Added description to update_issue, UpdateIssueParams, and clients
- Storage: description already supported in allowedUpdateFields

Tested in both daemon and direct modes.

internal/rpc/server.go

@@ -695,6 +695,9 @@ func updatesFromArgs(a UpdateArgs) map[string]interface{} {
 	if a.Title != nil {
 		u["title"] = *a.Title
 	}
+	if a.Description != nil {
+		u["description"] = *a.Description
+	}
 	if a.Status != nil {
 		u["status"] = *a.Status
 	}