feat(pager): document the Signal pager and add the receive timer

The pager worked but was undocumented and drifting stale: signal-cli warned the account had gone 47 days without a receive, and no runbook existed to hand a future session or a non-Claude runtime.

I added the runbook under docs/design/ and a roam-sync-shaped receive timer that drains the queue every 15 minutes, keeping the account warm and surfacing reply messages. I also generalized agent-page: it now sends directly from any machine holding the pager account and relays to velox only when it doesn't. ratio is linked as a device of the account, so a page lands even when velox is down.

The receive script no-ops cleanly where the account isn't registered, since the units stow onto every machine via the shared common package.

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