feat(triage-intake): add Signal source plugin

I added a Signal source plugin so the triage-intake sweep covers Signal alongside cmail, Gmail, calendar, and PRs. Signal is personal messaging, so it's a general plugin that syncs to every project.

It needs no wrapper script, unlike cmail. signal-cli is already a full CLI, so the plugin drives receive and send directly. The scan filters signal-cli's JSON down to real messages and drops the sync, receipt, and typing noise. One sharp edge is documented in the plugin: signal-cli receive drains the server queue, so the triage gets one shot per message. Signal Desktop and the phone keep their own copies, so nothing's lost.

I also added Signal to the engine's general-plugin list.

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