Feature Request: Dashboard & Widget Management Tools

## Summary

Add MCP tools for creating, editing, and managing Sentry dashboards and widgets.

## Motivation

We're building an internal agent skill to help teams set up and iterate on Sentry dashboards without manually navigating the UI. Dashboard management is a natural fit for an agentic workflow — teams could describe what they want to monitor and have an agent scaffold the dashboard for them.

Currently the MCP server has no tools for dashboard management, which blocks this use case entirely.

## Requested Tools

- `list_dashboards` — list dashboards in an organization
- `get_dashboard` — retrieve a dashboard and its widgets
- `create_dashboard` — create a new dashboard
- `update_dashboard` — rename or modify a dashboard
- `delete_dashboard` — remove a dashboard
- `create_widget` / `update_widget` / `delete_widget` — manage individual widgets within a dashboard

## Notes

Even a read-only `list_dashboards` + `get_dashboard` would be a helpful starting point. Full CRUD would unlock the agent-driven dashboard-building workflow described above.