fix(slack): harden notifications like signel's

cj/slack-notify passed raw message bodies to a bare start-process. A multiline message made a giant toast, and a machine without the notify script errored inside the condition-case, so the notification silently vanished. It now routes through the same shape signel uses: whitespace collapsed and the body truncated at 120 chars, sound gated by a new cj/slack-notify-sound defcustom (silent by default), and a notifications-notify fallback when the script is absent. The fallback requires notifications.el first, which isn't autoloaded. cj/slack-test-notify exercises the same delivery path.

The shared cj/messenger-notify extraction that collapses the signel and slack copies stays with the messenger-unification task. Ten tests cover the formatter, the delivery routing, and the DM/mention predicate wiring the audit called untested.

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