docs(messenger): add the per-app keybinding alphabet to the unification spec

I added a "Global Prefix Keybinding Alphabet" section to the messenger-unification spec. The per-app C-; prefix is a third keybinding surface, separate from the in-buffer chords (C-c C-c / C-c C-k / C-c C-a) and decision 6's cross-app verbs. Today the action leaf under each app is ad hoc: the same key means different things in Slack, Signal, Telega, and ERC. The section spells out the canonical actions, shows the inconsistency as a matrix, and proposes one leaf alphabet across all four, with the core seven verbs as the unifiable floor and the richer verbs as optional per-backend extensions.

I also added a smoke-first parity note to Phase 1 (build the controllable signel replacement to the capability floor, not its ceiling) and promoted the todo task to [#A] "Unify Signel and All Messengers into one UX" with a direct link to the spec.

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