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_client.py

@@ -427,6 +427,8 @@ class BdCliClient(BdClientBase):
             args.extend(["--assignee", params.assignee])
         if params.title:
             args.extend(["--title", params.title])
+        if params.description:
+            args.extend(["--description", params.description])
         if params.design:
             args.extend(["--design", params.design])
         if params.acceptance_criteria: