Normalize --acceptance flag for bd update command

- Update bd update to use --acceptance instead of --acceptance-criteria
- Update MCP client to use --acceptance flag
- Simplify SKILL.md documentation now that both commands use same flag

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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

@@ -424,7 +424,7 @@ class BdCliClient(BdClientBase):
         if params.design:
             args.extend(["--design", params.design])
         if params.acceptance_criteria:
-            args.extend(["--acceptance-criteria", params.acceptance_criteria])
+            args.extend(["--acceptance", params.acceptance_criteria])
         if params.notes:
             args.extend(["--notes", params.notes])
         if params.external_ref: