docs: add the notifyd implementation plan

Plan one of two for the notification daemon: the Go daemon and notifyctl in
their own repo. The renderer plan is separate because the daemon is working,
testable software on its own, exercised through notifyctl.

The policy is pure and tested without a bus; the store's expiry, dismissal
and eviction are separate lifetimes, which the store tests pin; and
test-notifyctl.sh runs the daemon and CLI on a private session bus against a
temporary runtime directory.

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