MCP tool calls now send _meta.progressToken, so servers that emit
notifications/progress (Meshy refine, creative-lab) report live progress.
The executor binds a per-call relay; the MCP handler forwards each
notification to tool_progress_callback as a throttled (<=1 per 500 ms,
final always) tool.progress event. Surfaces:
- gateway event stream + legacy TUI gateway: tool.progress
{tool_id, name, progress, total, percent, message} (contract registered)
- Desktop: running tool row shows '42% · <message>' beside its timer;
plugins subscribe with ctx.onEvent('tool.progress')
- CLI: the running tool spinner shows the same text
Documented in the Desktop plugin SDK guide.
