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/models.py

@@ -73,6 +73,7 @@ class UpdateIssueParams(BaseModel):
     priority: int | None = Field(default=None, ge=0, le=4)
     assignee: str | None = None
     title: str | None = None
+    description: str | None = None
     design: str | None = None
     acceptance_criteria: str | None = None
     notes: str | None = None