fix(signal): honor the cache-clear promise and mark empty fetches warm

Two picker-cache bugs from the config audit, verified against current code first.

The cache docstring promised clearing on signel-stop, but nothing cleared it, so a stale contact list survived a relink or reconnect. I advised a named clear function :after signel-stop, which makes the docstring true.

An empty listContacts result cached nil, indistinguishable from a cold cache, so a zero-contact account re-ran the picker's blocking fetch (up to the 3s timeout) on every open. The fetch now caches an empty sentinel and the picker reads through cj/signel--cached-contacts. One existing test pinned the nil behavior and now asserts the sentinel instead.

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