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/tools.py

@@ -175,6 +175,7 @@ async def beads_update_issue(
     priority: Annotated[int | None, "New priority (0-4)"] = None,
     assignee: Annotated[str | None, "New assignee"] = None,
     title: Annotated[str | None, "New title"] = None,
+    description: Annotated[str | None, "Issue description"] = None,
     design: Annotated[str | None, "Design notes"] = None,
     acceptance_criteria: Annotated[str | None, "Acceptance criteria"] = None,
     notes: Annotated[str | None, "Additional notes"] = None,
@@ -191,6 +192,7 @@ async def beads_update_issue(
         priority=priority,
         assignee=assignee,
         title=title,
+        description=description,
         design=design,
         acceptance_criteria=acceptance_criteria,
         notes=notes,