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.

integrations/beads-mcp/src/beads_mcp/bd_daemon_client.py

@@ -279,6 +279,8 @@ class BdDaemonClient(BdClientBase):
             args["assignee"] = params.assignee
         if params.title is not None:
             args["title"] = params.title
+        if params.description is not None:
+            args["description"] = params.description
 
         data = await self._send_request("update", args)
         return Issue(**(json.loads(data) if isinstance(data, str) else data))