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/protocol.go

@@ -72,6 +72,7 @@ type CreateArgs struct {
 type UpdateArgs struct {
 	ID                 string  `json:"id"`
 	Title              *string `json:"title,omitempty"`
+	Description        *string `json:"description,omitempty"`
 	Status             *string `json:"status,omitempty"`
 	Priority           *int    `json:"priority,omitempty"`
 	Design             *string `json:"design,omitempty"`