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/server.py

@@ -322,7 +322,7 @@ async def create_issue(
 
 @mcp.tool(
     name="update",
-    description="""Update an existing issue's status, priority, assignee, design notes,
+    description="""Update an existing issue's status, priority, assignee, description, design notes,
 or acceptance criteria. Use this to claim work (set status=in_progress).""",
 )
 @require_context
@@ -332,6 +332,7 @@ async def update_issue(
     priority: int | None = None,
     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,
@@ -350,6 +351,7 @@ async def update_issue(
         priority=priority,
         assignee=assignee,
         title=title,
+        description=description,
         design=design,
         acceptance_criteria=acceptance_criteria,
         notes=notes,