Service worker tools: background discovery (spec service-workers explainer)

The spec has a supplementary [service workers explainer](https://github.com/webmachinelearning/webmcp/blob/main/docs/service-workers.md): agents discovering/invoking tools on sites the user doesn't have open.

Ambitious, design-first: what would `webmcp-tools/sw` look like? Tool registration from a SW context (no `document`), JIT page wake-up, how the bridge advertises background tools. Comment with a sketch before any code.